What is replication and in what part of the cell cycle does it occur

In the S (synthesis) phase of Interphase.

Here is the chronology of Interphase:
-G1 (first growth) phase: cell grows, carries out life functions

-S (synthesis) phase: DNA replication- cell copies DNA

-G2 (second growth) phase: cell makes microtubules, prepares for mitosis

Good luck & hope I helped! =)