let's suppose it is a /20 then we would have addresses from 192.168.0.1 to 192.168.15.255 Now let's suppose it is a /21 then we would have addresses from 192.168.8.1 to
192.168.15.255
So both summaries encompass the networks we want to summarize but the second one is the most restrictive one as it only encompasses the networks we were asked to summarize and not others so it is the correct summary.
In fact just count the number of subnets which is 8 and find the exponent of 2 which is 8, that gives you 3 and to find out the summary mask just do /24 - 3 = /21.
if you make it in binary you'll find out the answer too:
the interesting octet is 3rd one so let's convert in binary.
8 0000 1000
9 0000 1001
1 0 0000 1010
1 1 0000 1011
1 2 0000 1100
1 3 0000 1101
1 4 0000 1110
1 5 0000 1111
I've bolded the bits in common there are 5 so 16+5=21 which is mask and address is
1 92.168.8.0