"the isolation of cell-cycle mutations in yeast was greatly facilitated by the use of mutations, which allow investigators to study the effects of mutations that would be lethal if expressed constantly"

The correct answer is - conditional mutation.

Explanation:

Conditional mutation is a type of mutation that has less severe or wild-type phenotype or traits that physically appears under particular permissive environmental conditions and a mutant phenotype under particular restrictive environmental conditions.

Thus, the correct answer is a conditional mutation.