Why do complementary base pairing reduce errors in dna

I think you mean how the complimentary base pairing reduces errors in DNA??

Its called the 3' to 5' "Proof-Reading" Exonuclease:

There's a 3' to 5' proof-reading exonuclease that works by scanning along directly behind the DNA polymerase, as it (the DNA Plymerase ) is adding new nucleotides to the growing strand. Its important to note that DNA polymerase works in a 5' to 3' direction, which is the opposite of how an exonuclease works. If the last nucleotide added by DNA polymerase is mismatched, then the entire replication holoenzyme (the exonuccleus) backs up, removes the last incorrect base, and attempts to add the correct base again. The enzyme is "3' to 5'" because it scans in the opposite direction of DNA replication, which must always be 5' to 3'. The mechanism of the proof-reading system offers an explanation as to why DNA replication must occur in this direction.

D. in the nucleus
Hope it helps!