The correct answer is (4) Fruit ripening.

Polygalacturonase (PG) is a cell wall–degrading enzyme that increases strongly during the ripening of climacteric fruits like tomato. Antisense RNA against the PG gene was one of the earliest genetic engineering strategies (e.g., in Flavr Savr–type tomatoes) to reduce PG mRNA and enzyme activity, thereby slowing pectin breakdown, softening, and over‑ripening. This directly targets the fruit ripening process to improve firmness and shelf life.

Explanation of options:

1. Seed setting
   PG antisense has been used mainly to modify fruit softening; seed setting is regulated by fertilization, hormonal balance (auxin, gibberellin), and proper floral/ovule development, not primarily by fruit PG. So this option is not correct in the standard context of PG antisense.

2. Herbicide resistance
   Herbicide resistance traits typically involve enzymes like EPSP synthase (glyphosate resistance), ALS/AHAS (sulfonylurea resistance), or detoxification enzymes. PG antisense does not confer herbicide resistance, so this is incorrect.

3. Viral resistance
   Viral resistance is usually achieved via pathogen‑derived resistance (coat protein, replicase genes), RNAi targeting viral genomes, or dominant resistance genes. PG antisense targets a host cell‑wall enzyme and is not a standard viral‑resistance strategy.

4. Fruit ripening – CORRECT
   By expressing antisense RNA against polygalacturonase, transgenic tomato lines show reduced PG activity, slower pectin degradation, firmer fruits, and delayed softening. This is a classic example of genetic manipulation aimed specifically at fruit ripening and post‑harvest shelf life.