Which number is not a common factor of 15, 45, and 90?
a) 3
b) 5
c) 10
d) 15

None, but 7 is not a common factor of 15,45,and 90

think about it. 

3 x 5=15   <<== that already eliminates it

5 x 3= 15  <<== that also eliminates it

10 x 9= 90 <<== eliminated

15 x 1= 15 <<== so all of them are eliminated as an answer

I’m not sure what sujevtbis this