As it is asking substring of the base 64 and the given numbers are binary, so we need to make it pair of 6,
Because 2^6=64,
So the pairs will be 101000 | 110011 | 011111 | 101001 | 101010 | 11
The last number has only 2 bits so we will use padding so it'll be 110000
So they will be 40 51 31 41 42 and 48
Which is equals to o z f p q w ( see the base table of 64)
so the substring is "fpq".