They were having (1)/ a birthday party at home (2)/ next week. (3)/No error (4)

Options

A. 1 They were having
B. 2 a birthday party at home
C. 3 next week.
D. 4 No error

Answer

A

Explanation

are going to have will replace were having because -
going to is used when we want to talk about a plan for the future.
Look at the examples given below:
We are going to have lunch after 2:00 p.m. They are going to launch it next month. Hence, They are going to have is the right usage