Gel electrophoresis is the techniques by which negatively charged DNA fragments separate according to their molecular mass and the principle says that lighter goes the maximum distance that means heavy DNA stops first as D is heavier it separates first while B at last.

Electrophoresis is defined as a technique of separation of the charged molecules of the substance in a solution, by subjecting them to an electric field. The technique helps to determine the number, amount and the mobility of the components in a given sample or separate them. It is also used to obtain information about the electrical double layer, surrounding the particles. Differential migration of the charged molecules in the presence of the electric field is the main principle of electrophoresis. The resolving power of electrophoresis was greatly improved by the introduction of a technique called zone technique. The zone electrophoresis performed by using gel is referred to as gel electrophoresis. 

Answer is option B i.e. "It must be cut by restriction endonucleases"
Electrophoresis is a method for separation of macro-molecules like DNA. Preparing the DNA for electrophoresis is an important step. DNA is "digested" or cut by restriction enzymes before being run on a gel so that DNA can be separated based on DNA size, fragment and length.

Answer is option D i.e. "Spooling"
At the last step of DNA extraction, purified DNA precipitates out after the addition of chilled ethanol. This DNA appears as collection of fine threads in the suspension which can be spool over the glass rod, this method is called as DNA spooling.

Agarose is the supporting structure in the cell walls of certain species of algae and which is released on boiling. Agarose is a polysaccharide polymer extracted from seaweed. Slabs of agarose gels are used for electrophoresis.
Thus, the correct answer is option A.

The first step in a Southern blot technique is to prepare the DNA mixture by breaking it into smaller fragments using a restriction enzyme.

There are special techniques for the introduction of recombinant DNA into host cells. Microinjection, biolistics, infection by disarmed pathogens like Agrobacterium or bacteriophage are examples of such techniques. 

Gel electrophoresis is a technique used to separate DNA fragments according to their size. DNA samples are loaded into wells (indentations) at one end of a gel, and an electric current is applied to pull them through the gel. DNA fragments are negatively charged, so they move towards the positive electrode. 

Gel electrophoresis is a method for separation and analysis of macromolecules (DNA, RNA and proteins) and their fragments, based on their size and charge. It is used in molecular biology to separate a mixed population of DNA and RNA fragments by length, to estimate the size of DNA and RNA fragments or to separate proteins by charge.

A restriction enzyme (or restriction endonuclease) is an enzyme that cuts DNA at or near specific recognition nucleotide sequences known as restriction sites. The DNA fragments are separated by electrophoresis, a process that involves application of an electric field to cause the DNA fragments to migrate into an agarose gel. The gel is then stained with a methylene blue stain to visualize the DNA bands and may be photographed. 

CRO I has restriction sequence CAGGAC and ERA I has the restriction sequence GTAATG. When both the enzymes are used to cleave the bacterial and the human DNA, there are three fragments produced. So, only three bands will show up on the agarose gel as the restriction enzymes will cut very close. Thus, the correct answer is option C.