Which of the following tools/techniques would allow a researcher to measure the expression of a gene of interest in live organisms?

A) a lacZ reporter fused to the gene promoter
B) homologous recombination to replace the promoter of the gene of interest with a bacterial promoter
C) measuring the frequency of mutations by DNA sequencing
D) a transgene that fuses GFP to the gene of interest
E) RNAi targeting the promoter of the gene of interest