We have 3*8 decoder..
Now with 3 input lines we can make any one output line as active ...like
Say we have 111 then output line O7 will be activated in decoder.
Now From O0 to O7 we have 8 output lines at decoder which are connected to memory module .
So working is like If we generate one address then few bits are used to make one of memory module as active.
But as per condidtions in question 2 bits are needed to make decoder active .
So total case is like
out of 16 bits ...
2 bits always dedicated to make DECODER ON ..now again 3 bits are given to make any one memory moduel active ...
So total remaining bits are 16 - 2- 3 = 11 bits .
Means 11 bits are used to refer one word from one module ..so size of block will be 2 ^ 11 that is 2Kb