Final answer:
The critical factor for DNA replication on the leading strand is the primer.
Step-by-step explanation:
The critical factor for DNA replication on the leading strand is primer. The primer provides the initial starting point for DNA polymerase to begin adding nucleotides and synthesizing the new DNA strand. DNA polymerase, deoxynucleoside triphosphates, DNA ligase, and DNA primase are all essential components for DNA replication and are required on both the leading and lagging strands.