Which part of the DNA codes for proteins?

The correct choice refers to "exons," which are the portions of DNA (and corresponding RNA) that are expressed and ultimately code for proteins. During the process of transcription, the entire gene is transcribed into a precursor messenger RNA (pre-mRNA) that includes both introns (non-coding regions) and exons. However, during RNA processing, introns are removed, and the exons are spliced together to form the mature mRNA. This mature mRNA is what gets translated into the amino acid sequence of a protein.

Exons contain the actual coding information needed to synthesize proteins. Thus, when referring to the part of DNA responsible for coding proteins, exons are the critical components that fulfill this function. Understanding the role of exons in protein coding is essential for grasping the basic principles of molecular biology and genetics.