3+x=7 What does variable represent?

an unknown number by the way the answer is 4 too.

good morning here is your answer,

3+x=7

or, x= 7-3

or, x= 4

1) 2/5

2) zero=0/1

3) zero=0/2

4) -4/5

5) undefined=3/0

the answer is b