commonly used barometer to measure pressure is filled with mercury such that 1 atm = 76 cm of Hg .

Barometer is the instrument used to measure atmospheric pressure.

 pressure = $\rho$gh 

torricellian vacuum is the vaccum formed above the column of mercury in a barometer.

Torricellian vacuum
The vacuum formed when a long tube, closed at one end and filled with mercury, is inverted into a mercury reservoir so that the open end of the tube is below the surface of the mercury. The pressure inside the Torricellian vacuum is the vapour pressure of mercury, about $103\ torr.$

Torricellian vacuum
The vacuum formed when a long tube, closed at one end and filled with mercury, is inverted into a mercury reservoir so that the open end of the tube is below the surface of the mercury. The pressure inside the Torricellian vacuum is the vapour pressure of mercury, about 103 torr.

The instrument that measures air pressure is a barometer.  There are two types of barometers, mercury and aneroid. In 1643, Toricelli invented the mercury barometer when he inserted a glass tube in a dish of mercury and observed the day-to-day variations in the height of the mercury column. When the atmospheric pressure was high, the greatest pressure was exerted on the mercury in the dish, causing the mercury in the glass tube to rise.  When the pressure was low, the mercury in the glass tube fell. The contemporary aneroid barometer employs a vacuum chamber sensitive to external pressure changes.  Contraction and expansion of the vacuum chamber, linked mechanically to an indicator needle, move the needle clockwise when the pressure rises and counter clockwise when the pressure falls.

Answer is C.

The mercury level in the tube of the barometer falls until it is about $76 cm (760 mm)$ vertically above the mercury level. It is the atmospheric pressure acting on the surface of the mercury in the trough that supports the vertical mercury column.
The atmospheric pressure is therefore specified by so many millimetres of mercury. For example, the atmospheric pressure is $760 mm$ Hg at sea level.
Hence, The vertical height of mercury which a simple barometer can support at sea level is less than 76 cm. 

A barometer is an instrument that is used to measure pressure.

A barometer is a device used for measuring atmospheric pressure.

A classic mercury barometer is constructed from a 3-foot glass tube that is open at one end and sealed at the opposite end. The glass tube is filled with liquid mercury and rests upside-down in a reservoir of mercury. As the mercury moves down the glass tube, a vacuum is created. When the atmospheric pressure above the reservoir increases, the mercury inside the tube rises. As atmospheric pressure decreases the mercury moves down the tube, into the reservoir.The change in the mercury level in the glass tube is equal to the pressure exerted by the air above the reservoir and is measured using the scale marked on the glass tube. 

A manometer is any device used to measure pressure. However, the word manometer is commonly used to mean a pressure sensor which detects pressure change by means of liquid in a tube.
Manometers are differential pressure sensors. A differential pressure sensor measures the difference between a pressure being applied to it and a reference pressure (often atmospheric pressure). 

Answer is A.

A barometer is a scientific instrument used in meteorology to measure atmospheric pressure. A simple barometer consists of a long glass tube (closed at one end, open at the other) filled with mercury and turned upside down into a container of mercury. The barometer works by balancing the mercury in the glass tube against the outside air pressure, just like a set of scales. As air pressure increasesthat is, as the air becomes heavierit pushes more of the mercury up into the tube. As air pressure decreases, more of the mercury drains from the tube. So the level of mercury in the tube provides a precise measure of air pressure.

• The mercury Barometre was invented by Torricelli.

The tube in a simple barometer is 'completely' filled with mercury so that no air bubble is left inside the tube. 

The height to which mercury rises in simple barometer is $76cm$.

A mercury barometer was discovered by Torricelli in the year 1643. 

The mercury column in the barometer fall rapidly before a severe storm, It is due to Fall in the atmospheric pressure. In general, a falling barometer indicates the approach of a storm. If the mercury is over 30.20 inches but falling quickly, warmer, cloudier weather is coming 

Tilting of tube does not brings any change in height  as  pressure depends on height and as it is not changing. 

In summer, when the barometer falls suddenly, a thunderstorm can be expected, and if it does not rise again upon its cessation, the weather will probably continue unsettled for several days. In summer, when a thunderstorm happens, there is little or no depression of the barometer.

The tube contains air $($because it is not fully filled$).$ This air pressure against atmosphere pressure$,$ 

$L _0=75$cm

FALSE
A simple barometer is neither portable nor compact. Thus, it cannot be carried from one place to other.
Since the apparatus is made up of glass, so there is also a chance of breaking.

$P = \rho gh$, where
$P = $air pressure $= 101325Pa$
$\rho = $density of mercury$ = 13594kg/{m}^{3}$
$\implies h = 0.76m$

If water would be used in barometer, barometer tube should have the huge height of $10.3m$ Mercury is $13.6$ times more dense than water and a column weight of only $760 mm$ of Hg equals the forces generated by atmospheric pressure, so the tube can be much shorter.

Mercury is more dense. Its relative density is $13.534$ times that of water. So, to measure the atmospheric pressure, which is $760 mm$ of mercury you need a barometer ,with mercury, of length slightly more than $0.76 m$, say one meter to cover the special cases of some higher pressure. If you use water, you have to have the length of barometer of length (or height)  $13.534$ times the length of mercury barometer, which may be more than $11 m$ in length. Further mercury, being a metal has the shining quality which highlight its reading clear. Also mercury, having comparatively lower specific heat and good conductor of heat, could come to the same temperature of the atmosphere more quickly.

Mercury is more dense. Its relative density is $13.534$ times that of water. So, to measure the atmospheric pressure, which is $760 mm$ of mercury you need a barometer ,with mercury, of length slightly more than $0.76 m$, say one meter to cover the special cases of some higher  pressure. If you use water, you have to have the length of barometer of  length (or height)  $13.534$ times the length of mercury barometer, which  may be more than $11 m$ in length. 

Answer is A

Mercury barometer was invented by scientist Evangelist Torricelli

The movement of elevator up will increase value of g which will result in increase in pressure leading to increase in reading of barometer.

${ P } _{ exp }=P _{ 0 }+{ P } _{ Actual }\ 190=76+{ P } _{ Actual }\ { P } _{ Actual }=114mm\quad of\quad Hg\ Factor=\cfrac { { P } _{ Actual } }{ { P } _{ atm } } =\cfrac { 114 }{ 76 } \approx 1.25$

Since the elevator is accelerating up, so, the force will act in the downward direction.

Since there is no atmosphere on moon, no atmospheric pressure will be there and hence corresponding to 0 pressure, height of mercury column will be 0.

Pressure with Height: pressure decreases with increasing altitude. The pressure at any level in the atmosphere may be interpreted as the total weight of the air above a unit area at any elevation. At higher elevations, there are fewer air molecules above a given surface than a similar surface at lower levels. For example, there are fewer molecules above the 50 km surface that are found above the 12 km surface, which is why the pressure is less at 50 km.

Hence, Atmosphere pressure decrease with an increase in altitude 

Due to present difference.

As we go to higher attitudes, atmospheric pressure decreases due to less air molecules on comparision to that at sea level. 

Atmospheric pressure decreases with increase in height & pressure at a point at height h.

$\begin{array}{l} \therefore \, \, { V _{ rms } }=\sqrt { \dfrac { { 3PV } }{ M }  }  \ E=\dfrac { 1 }{ 2 } M\, \, { V _{ rms } }^{ 2 }=\dfrac { 3 }{ 2 } PV=\dfrac { 3 }{ 2 } P\, \, \left[ { \therefore \, \, V=1 } \right]  \end{array}$