For OS numericals, conversion of memory size in terms of 2 powers is very important
So Disk size = 1TB = 2^40 B
Block Size = 4KB =2^12 B
No of blocks = (2^40 ) / ( 2^12) = 2^28 blocks
Now for each block to represent in bit map we will require 1 bit so bitmap size in bits = 2^28 bits (to represent 2^28 blocks)
So bitmap size in Bytes = 2^28 / 2^3 =2^25 = 32 MB
P.S Thanks @Harish Kumar you gave me more clarification on this question