Final answer:
In dehydration synthesis, molecules combine by forming a covalent bond and releasing water, as seen in peptide bond formation between amino acids. Therefore, the correct option is A.
Step-by-step explanation:
In dehydration synthesis, a hydrogen atom from one molecule combines with a hydroxyl group (OH) from another molecule, causing the two molecules to form a covalent bond and release a water molecule. This process is exemplified in the formation of a peptide bond between amino acids, which is a typical dehydration synthesis reaction. When the carboxyl group of one amino acid pairs with the amino group of another, they release water and create a peptide bond that links the amino acids together, thus forming proteins.