The cytoplasm- genetic male sterility and fertility restoration system using three lines, male sterility (A), maintainer (B), and restorer (R) is being used in many croplands for the production of hybrid seed. Identical nuclear genes are possessed by.
A. A and B lines
B. A and R lines
C. B and R lines
D. A, B, and R line

Hint: Identical nuclear gene is produced between isogenic lines. Hybrid seed production using CMS (Cytoplasmic Male Sterility) is based on the three-line system i.e. A, B, and R lines.

Complete answer:
The CMS line is called A-line which is male sterile. It is sterile because the genes are in the cytoplasm (i.e mitochondrial DNA).
The Maintainer line is called the B line. It is isogenic to CMS (A-Line) except for fertility.
Isogenic means they are genetically identical except for one gene.
It helps in maintaining A-line.
A line does not have any active male part as it is sterile so to maintain it we want the seed of A-line to cross with B line to get a product.
The restorer line is called the R line. It has a restorer gene in its nucleus to restore the fertility of the A-line.
The R line is not similar to that of the A and B lines. It is used to produce hybrid seeds because it has a very high SCA (Specific Combining Ability).
Therefore to produce identical nuclear genes we need to combine the CMS line with the Maintainer line.

Hence, the correct answer is option A) A and B lines.

Note:When B line is crossed with B line- B line seeds can be produced.
When the R line is crossed with the R line- the R-line seeds can be produced.
When A line is crossed with B line- A line seed can be produced.
When A line is crossed with the R line- a Hybrid of the seed can be produced.