Explanation:

- Chromosome number 21: In a normal human, chromosome 21 forms a homologous pair. Both males and females have two copies.

- Chromosome number: This seems incomplete, but likely points to an autosome similar to 21 or 13, which also forms pairs in both males and females.

- X-chromosome: In a normal human male, the X and Y chromosomes form a mismatched pair as they are different from each other.

- Chromosome number 13: Like 21, it forms a homologous pair in a normal human.

Correct Answer: Option:3, X-chromosome.