Final answer:
DNA ligase is the enzyme used to covalently link two DNA fragments to form a stable recombinant DNA molecule.
Step-by-step explanation:
The enzyme used to covalently link two DNA fragments to form a stable recombinant DNA molecule is DNA ligase. DNA ligase catalyzes the formation of a covalent phosphodiester linkage between the 3'-OH end of one DNA fragment and the 5' phosphate end of another DNA fragment. This process is essential for the creation of recombinant DNA molecules in genetic engineering.