Various types of skin colour in humans is an example of:

The correct answer is Option (3) → Polygenic inheritance

Option (1) Pleiotropy — Incorrect. Pleiotropy is one gene affecting many traits.

Option (2) Incomplete dominance — Incorrect. In incomplete dominance one gene gives an intermediate phenotype.

Option (3) Polygenic inheritance — Correct. Several genes add their effects to give a range of skin colours.

Option (4) Co-dominance — Incorrect. In co-dominance both alleles show fully, as in the AB blood group.

NCERT Extract:
Polygenic Inheritance – “… Such traits are generally controlled by three or more genes and are thus called as polygenic traits. Besides the involvement of multiple genes polygenic inheritance also takes into account the influence of environment. Human skin colour is another classic example for this. In a polygenic trait the phenotype reflects the contribution of each allele, i.e., the effect of each allele is additive.”