Final answer:
A merozygote, or partial diploid, is an organism or cell with a full set of chromosomes from one parent and additional genes from the other, without complete duplication of the entire genome. Therefore, the correct answer is that a merozygote is a partial diploid.
Step-by-step explanation:
A merozygote, also known as a partial diploid, is an organism or cell that has a full set of chromosomes from one parent and an additional set or partial set of genes carried on a plasmid or a fragment of a chromosome from the other parent. This situation is not a complete diploidy, hence option A is incorrect. Also it's not unrelated to diploidy because it involves an atypical diploid state; thus option B is incorrect.
It's not necessarily a result of a mutation, so option D would be misleading. Therefore, the correct answer to the question is option C, which states that a merozygote is a partial diploid. In contrast to polyploidy, merozygosity typically does not involve the entire genome being duplicated. Rather, it involves extra copies of specific genes or gene clusters.