Which one of the following phenotypic ratios is correct for dihybrid…

The correct answer is Option (4) → 9 : 3 : 3 : 1

A dihybrid cross follows two characters at a time, for example seed shape (round R / wrinkled r) and seed colour (yellow Y / green y). Each character alone gives a 3 : 1 ratio in the F₂. The two characters are inherited independently, so the two ratios combine:

    (3 round : 1 wrinkled) × (3 yellow : 1 green)
    = 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green

So the phenotypic ratio is 9 : 3 : 3 : 1, out of 16.

(1) 9 : 1 : 3 : 3 — Incorrect. The numbers are the same, but the order is wrong. The 1 belongs to the class that shows both recessive traits, which is written last.

(2) 3 : 7 : 3 : 3 — Incorrect. No such class of 7 appears in the cross.

(3) 3 : 2 : 2 : 9 — Incorrect. This does not come from combining two 3 : 1 ratios.

(4) 9 : 3 : 3 : 1 — Correct. This is the dihybrid phenotypic ratio.

NCERT Extract:
Law of Independent Assortment –

Figure 4.7 Results of a dihybrid cross where the two parents differed in two pairs of contrasting traits: seed colour and seed shape