Assertion (A) : S or synthesis phase marks the period during which the amount of DNA per cell doubles.
Reason (R) : If the cell is diploid (2n at  $G_1$), after S-phase at $G_2$ ,the number of chromosomes becomes 4n. 

S or synthesis phase marks the period during which DNA synthesis or replication takes place. During this time the amount of DNA per cell doubles. If the initial amount of DNA is denoted as 2C then it increases to 4C. However, there is no increase in the chromosome number; if the cell had diploid or 2n number of chromosomes at G1 , even after S phase the number of chromosomes remains the same, i.e., 2n.