In single cross hybrid:

Correct option is A

A single cross hybrid involves a cross between two inbred lines to produce F₁ hybrid seed. When using cytoplasmic male sterility (CMS) technology to produce hybrid seeds (especially in crops like maize, rice, and sorghum), the following components are used:

1. A-line (Male sterile line): This line carries cytoplasmic male sterility and cannot produce viable pollen.
2. R-line (Restorer line): This line carries fertility restorer genes that can restore fertility in the hybrid progeny.

In single cross hybrid seed production, the male sterile (A-line) is crossed with the restorer line (R-line) to produce F₁ hybrid seeds. These seeds are fertile and high-yielding, suitable for commercial cultivation.

Why Other Options Are Incorrect:

• (b) Male sterile line is crossed with maintainer:
  This cross is used only to maintain the male sterile line (A-line), not to produce hybrids.
• (c) Maintainer is crossed with restorer:
  This cross does not produce hybrid seed or serve a standard breeding purpose.
• (d) Restorer is crossed with male fertile line:
  This is not relevant in hybrid seed production since it does not involve the male sterile line.