Answer:
Please be quiet! The baby is sleeping.
Ruba saw a doctor today.
Explanation:
First, a. At the beginning of a, they write "please be quiet!" This means that the baby was not sleeping in the past, but the the baby is sleeping in the present, right now. Why would they say be quiet if the baby already slept? So, a is in present tense, so you you use "is."
Now, b. It is most probable that Ruba already saw the doctor earlier today so we use "saw" for past tense. Ruba saw a doctor today.
