Final answer:
In microarray procedures, DNA is the molecule that is labeled with a fluorescent tag to visualize hybridization to complementary sequences.
Step-by-step explanation:
In the microarray procedure, the molecule that is labeled with a fluorescent tag is DNA. This technique involves using a probe, which is a short piece of DNA complementary to a sequence of interest. The probe is attached to a fluorescent dye, enabling the visualization of the DNA fragments to which it hybridizes. When genomic DNA or mRNA is used for a microarray experiment, the mRNA is converted to complementary DNA (cDNA) using reverse transcriptase. Subsequently, this DNA or cDNA is tagged with different fluorescent dyes and combined with the microarray chip to allow for hybridization to complementary sequences.