Final answer:
The molecule commonly measured to detect gene expression is mRNA, denoting the activity level of a gene. Telomerase maintains the ends of linear chromosomes, and DNA ligase is implicated in cases where the joining of Okazaki fragments is impaired.
Step-by-step explanation:
The molecule that is commonly measured to detect the level of gene expression is b) mRNA. During gene expression, DNA is transcribed into mRNA, which is then translated into proteins. The amount of mRNA produced from a gene can be used as an indication of how active that gene is, thus serving as a measure for gene expression. In terms of the molecule that maintains the ends of linear chromosomes, the correct answer is d) telomerase. Regarding the enzyme that may be mutated when the joining of Okazaki fragments is impaired, the answer would be DNA ligase. A promoter is a specific DNA sequence to which RNA polymerase binds to initiate transcription.