For the sentence, "it is raining," we have to use the verb estar. Why?
Estar is used mainly to indicate locations and temporary situations or conditions. This verb is used with locations, temporary physical conditions, temporary mental conditions and moods, and with the present progressive.
The problem is asking for the translations of "it is raining." Note that the word "raining" ends with an -ing. This is present progressive.
In Spanish, the verb "to rain" is llover. The present participle (progressive construction) of llover is lloviendo.
Since we are talking about weather here, we have to use the third singular form of estar which is "está."
So, the translation for "it is raining," is:
Está lloviendo.