In the fourth semester (1)/ of the course, the (2)/ attendance fell down.(3)/ No error (4)

Answer

3

Explanation

fell will replace fell down because -
fell (Verb) : to decrease
fell down (Verb) : to drop down from a higher level to a lower level
Look at the examples given below :
He fell down and fractured his wrist.
Their profits have fallen by 30 per cent.
Hence, attendance fell is the right usage.