What does $S$ represent in the given diagram?

Given below are the two statements.
Statement $I$: Replication of $DNA$ occurs in the '$S$' phase of interphase in the cell cycle.
Statement $II$: Replication of $DNA$ takes place only once in the cell cycle.
In the light of above statements,choose the correct answer from the options given below:

In which phase does the synthesis of spindle fiber proteins occur?

Which of the following events does not occur during interphase?

In which phase does $DNA$ replication occur?

Which of the following human diploid cells do not undergo cell division?