After an RNA primer is replaced by DNA, a nick in the DNA backbone is sealed by: A) 5'-3'exonuclease
B) DNA topoisomerases
C) DNA ligases
D) 3-5' exonuclease

C

Explanation:

enzyme DNA ligase

The nicks that remain between the newly synthesized DNA (that replaced the RNA primer) and the previously synthesized DNA are sealed by the enzyme DNA ligase, which catalyzes the formation of phosphodiester linkages between the 3′-OH end of one nucleotide and the 5′ phosphate end of the other fragment.