How can researchers distinguish exons from introns in a segment of dna?
a. the sequence of exons complements mrna molecules in the cell.
b. only exons contain three-base sequences that can code for amino acids.
c. primers won't bind to introns.
d. exons have a characteristic sequence.

a. The sequence of exons complements mRNA molecules in the cell.

Explanation:

It is only sequence of exons which complement mRNA molecules which are present in the cell. The intron sequences are removed during processing of mRNA. Once an mRNA is synthesized from template DNA strand, the introns are removed from the pre mRNA by a process named as RNA splicing. After removal of introns, the processed mRNA is left with only those sequences which correspond to exons. So when a processed mRNA will be allowed to complement with the template DNA strand, only exon sequences of DNA will match with those of spliced mRNA sequences.  

Note: Prokaryotic mRNAs rarely undergo splicing, it is only eukaryotic mRNAs which undergo splicing.