Aconditional mutation is one that allows a mutant gene product to function normally under the condition, but to function abnormally under the condition. such mutations are especially useful for the study of mutations.

A conditional mutation is one that allows a mutant gene product to function normally under the permissive condition, but to function abnormally under the restrictive condition. Such mutations are especially useful for the study of   lethal mutations.

Explanation:

A conditional mutation can be described as a form of a lethal mutation as it's effects can be devastating. This type of mutation functions like a wild type mutation under the normal conditions. When the condition becomes restrictive, these kind of mutations start to show their effect on the phenotype of the organism. The conditional mutations can cause an organism to not be able to survive under certain conditions, which eventually leads to the death of the organism.

the answer is c) an oak tree

that is wrong, its not thymus.