Gel electrophoresis is used for

The correct option is A Separation of DNA fragments according to their size
Gel electrophoresis
Gel electrophoresis is the technique involving the separation of molecules based on their size and charge. For example, it is used for the separation of DNA fragments formed after treatment with endonucleases, based on their size and negative charge.

Process of gel electrophoresis
The process of agarose gel electrophoresis is as follows:

• The DNA fragments and size markers are loaded onto the gel well and current is passed through the gel.
• The DNA is negatively charged, and moves towards the positive electrode during gel electrophoresis.
• The smaller the size of the molecule, the faster and farther it travels and the fragments are separated.

DNA ligase
DNA ligase creates the phosphodiester bond between the nucleotides of DNA fragments and joins them together. It helps in the joining of vectors and DNA of interest in genetic engineering.

Isolation of genetic material
The isolation of the genetic material from the cell is done in the following way:-

• Breaking the cell wall - Enzymes are used to break the cell wall depending on the species. The choice of the enzyme is based on the component of the cell wall.
• Removal of proteins (histones) - DNA molecules are intertwined with histone proteins, which can be removed by treating them with enzyme protease.
• Removal of RNA - To obtain DNA devoid of RNA, the sample is treated with ribonucleases.

Restriction endonuclease
Restriction endonucleases are the enzymes that cleave the DNA by digesting phosphodiester bonds at the specific sequences within the DNA. Examples include $Eco$R I.

Final answer
(D) Separation of DNA fragments according to their size