In diploid species, diploid cells may undergo either mitosis or meiosis. Haploid cells may undergo mitosis (for certain species) but NOT meiosis because

The correct answer would be - Homologous chromosomes cannot pair

Explanation:

Haploid cells are the cells that have single set of chromosome or have the equal number of chromosome as their parent cells. Haploid cells are denoted by the n. Haploid cells are produced by the meiosis and are identical copies of their parent cells,

Haploid cells may undergo mitosis but not meiosis as their are already n number of chromosome and homologous chromosomes are can not pair and meiosis is a reduction division that produces haploid cell from diploid cells.

Thus, the correct answer is - Homologous chromosomes cannot pair