Correct answer: (a) Aneuploidy

Explanation:

Failure of segregation of chromatids during cell division cycle results in the gain or loss of a chromosome(s), called aneuploidy. Aneuploidy arises due to non-disjunction of homologous chromosome.

Aneuploidy is of four types:

1. Monosomy = 2n – 1

2. Nullisomy = 2n – 2

3. Trisomy = 2n + 1

4. Tetrasomy = 2n + 2