Flower colour in Antirrhinum sp. is an example of:

The correct answer is Option (3) → Incomplete dominance

Option (1) Mendel's dominance — Incorrect. In Mendel’s dominance the F₁ looks like one parent.

Option (2) Co-dominance — Incorrect. In co-dominance the F₁ resembles both parents, as in the ABO blood groups where I^A and I^B both show. In snapdragon the F₁ is pink, in between red and white, so it is not co-dominance.

Option (3) Incomplete dominance — Correct. In Antirrhinum (snapdragon), a cross between red and white flowers gives pink F₁ flowers. Neither colour is fully dominant, so the heterozygote is in between; this is incomplete dominance.

Option (4) Multiple allelism — Incorrect. Multiple alleles are three or more forms of a gene, as in the ABO blood groups.

NCERT Extract:
Incomplete Dominance – “… In a cross between true-breeding red-flowered (RR) and truebreeding white-flowered plants (rr), the F1 (Rr) was pink (Figure 4.6). When the F1 was self-pollinated the F2 resulted in the following ratio 1 (RR) Red : 2 (Rr) Pink : 1 (rr) White.”