The answer is DNA polymerase.
This is the editor enzyme for DNA replication.
This enzyme catalyzes the replication of DNA, it checks the nucleotide sequence by "proofreading" every base it adds. If the enzyme detects that a wrong base was added, it changes the nucleotide right away to the right one before adding another. This process is highly accurate, as only 1 mistake is made for every 10⁹ bases.