Ais a segment of a chromosome that codes for a specific protein.
a: gene
b: crossed out
c: amino acid
d: nucleotide

In the sentence "A is a segment of a chromosome that codes for a specific protein", the word that correctly completes the sentence is A: gene. In other words, a gene is a sequence of a chromosome that holds the code for protein synthesis. In the transcription, the gene is transcribed into messenger RNA (mRNA). In translation, mRNA is translated into proteins.