• Assertion (A) is True: Genetic Male Sterility (GMS) lines cannot produce viable pollen and therefore cannot self-pollinate. To maintain the GMS line (female parent), it must be continually maintained by backcrossing with an identical heterozygous male-fertile maintainer line ($Ms/ms$), which yields a 1:1 segregating progeny of fertile ($Ms/ms$) and sterile ($ms/ms$) plants.
• Reason (R) is True: The male sterility phenotype in monogenic GMS is governed by a recessive nuclear gene ($ms$). Because the homozygous recessive genotype ($ms/ms$) is completely sterile and the maintainer parent provides the dominant allele ($Ms$), heterozygous crossing maintains the recessive allele in the population.
• Relationship: Because the recessive nuclear gene ($ms$) governs the phenotype, cross-pollination with heterozygous plants ($Ms/ms$) is biologically mandatory to generate sterile segregants in every maintenance generation. Thus, Reason (R) correctly explains Assertion (A).
• Conclusion: Both (A) and (R) are true and (R) is the correct explanation of (A) (Option 1).

Correct option: (1) Both (A) and (R) are true and (R) is the correct explanation of (A)