The elements of the triads had similar properties and when arranged in the increasing order of atomic masses, the atomic mass of the middle element was roughly the arithmetic mean of the masses of the other two elements.

Modern periodic table is based on atomic number.

Magnesium resembles beryllium according to Newlands’ law of octaves.

According to Newlands' law of octaves, when the elements are arranged in order of increasing atomic weights, then every eighth element has properties similar to that of the first element.

In Mendeleev’s periodic table, cobalt and nickel were placed in an inverted sequence as nickel and cobalt.

The vertical columns in a periodic table are called groups.

The element ‘eka-boron’, predicted by Mendeleev, is scandium.

Option (3) is correct.

Both a and c are wrong. Mendeleev classified the 56 known elements of his time. No separate slot was alloted to the isotopes in Mendeleev's periodic table.  

He has the lowest electronegativity.

 Li < Na < K < Rb < Cs

Metallic character decreases in a period and increases in a group.

Caesium (Cs) has the greatest electropositive nature among all the given elements because as we move across a period, electropositive character decreases whereas down a group, it decreases.

As we move from one element to the next in a specific period in the modern periodic table, the atomic number  increases by one.

As we move from left to right across the periodic table, the protons get added to the nucleus and electrons are added to the same shell. Hence, the effective nuclear charge of atoms increases.