Being a rainy day (1)/ we didn't (2)/ go out. (3)/ No error (4)

Answer

1

Explanation

It being a rainy day will replace Being a rainy day.
This is an error related to Unattached Subject/Par- ticiple.
Participles are used inabsolute phrases with a Noun/ Pronoun going before them.
A Participle is a Verb - Adj. It should be related to a proper Subject of reference.
Look at the examples given below:
Driving down the road, a leopard leapt out in front of me.
This sentence means that it was the leopard which was driving down the road
As was driving down the road, a leopard leapt outin front of me