A condition, where a certain gene is present in only a single copy in a diploid cell is called:

The correct option is D

Hemizygous

The correct option is D:

Explanation of the correct option:

1. Hemizygous is the condition of having a single allele.
2. An example of a hemizygous condition is the presence of genes on the X-chromosome in males who have only one X chromosome only.

Explanation of the incorrect options:

Option A:

Heterozygous is the condition of having both dissimilar alleles or different alleles.

Option B:

The relationship of spending complete life with a single partner is called monogamy.

Option C:

Homozygous is the condition of having both similar alleles in a diploid cell.

Final answer: A condition, where a certain gene is present in only a single copy in a diploid cell is called hemizygous.