The greatest common factor of 16 and the positive integer n is 4=> n and 16 have 4 as their factors
=> n is a multiple of 4 and can be a multiple of some other number which does not have any common factor with 16
=> n is a multiple of 4 and can be a multiple of any other odd number or numbers ...(1)
The greatest common factor of n and 45 is 3=> n and 45 have 3 as their factors
=> n is a multiple of 3 and can be a multiple of some other number which does not have any common factor with 45 ...(2)
Combining (1) and (2) we get n has 3 and 4 as its factors=> n is a multiple of 3 and 4 => n is a multiple of 12 => n = 12k
GCD(n,210)
=> GCD(12k,210)
Now 12 and 210 have only 6 common but 6 is not there in the answer choice. So we will look for multiples of 6 and see which are possible
Only C and D are possible
C is 30 but n cannot have 5 as its factor because n and 45 have only 3 common
So,
Answer will be DHope it helps!
To learn more about LCM and GCD watch the following videos