Correct option is D
✅ (d) A-line: • A-line is male sterile, used in hybrid seed production. • Cannot self-pollinate; ensures hybrid vigor. • Maintained by crossing with B-line. • Essential for commercial hybrid programs. ❌ (a) R-line: • Restorer line, used to restore fertility. ❌ (b) B-line: • Maintainer line, used to multiply A-line. ❌ (c) C-line: • Not standard terminology in hybrid seed production.