Help me please

Ah, many answers are correct, it depends on the point of view...

1. Snapshot in the present
"she is writing letters while her son is watching TV"
Both are doing this at the same time; the duration is the same.

2. Snapshot in the past
"she was writing letters while her son was watching TV"
Both were doing this at the same time; same duration.

3. "she wrote letters while her son was watching TV"
implies that when she finished her letters her son was still watching TV (the -ing occupation lasts longer than the one in the simple past)

4. "she was writing letters while her son watched TV"
implies that her son stopped watching TV before she finished her letters, for the same reasob as in 3.