A compensation loop is formed when homologous chromosomes pair during meiosis. It is a mechanism that allows for the equalization of gene dosage between the homologues. This process occurs during certain events that alter the structure of the chromosomes. Let's explore each option to understand when a compensation loop would form:

a) Deletion:
- During a deletion event, a segment of the chromosome is lost.
- If a deletion occurs in one of the homologous chromosomes, it can result in an imbalance of gene dosage between the homologues.
- In order to compensate for this imbalance, a compensation loop may form between the intact region of one homologue and the corresponding region of the other homologue.

b) Inversion:
- In an inversion event, a segment of the chromosome is flipped and reinserted in the opposite orientation.
- Inversions do not typically result in an imbalance of gene dosage between homologous chromosomes.
- Therefore, a compensation loop is not necessary during an inversion event.

c) Duplication:
- During a duplication event, a segment of the chromosome is duplicated.
- This can result in an increased dosage of genes in one of the homologous chromosomes.
- To equalize the gene dosage between the homologues, a compensation loop may form.

d) Translocation:
- Translocation occurs when a segment of one chromosome breaks off and attaches to another non-homologous chromosome.
- Translocations can result in an imbalance of gene dosage between the homologous chromosomes.
- Therefore, a compensation loop may form to equalize the gene dosage.

In summary, a compensation loop would form during events that result in an imbalance of gene dosage between homologous chromosomes, such as deletion and duplication. Inversions do not typically cause gene dosage imbalances, and translocations can result in imbalances, leading to the formation of a compensation loop.