These poisonous gases (1)/ will effect (2)/ our health. (3)/ No error (4)

Answer

2

Explanation

affect will replace effect because -
affect (Verb) : to produce a change
effect (Verb): a change that occurred
Look at the examples given below :
The weather conditions will affect the number of people who come to the country fair this year, The new manager effected some positive changes in the office.
Hence, will affect is the right usage