Using BITAND(1,1) = 1 and BITAND other three combinations being 0 we can obtain 
the function output as follows.

For BITAND(1,1) the binary value of AND(01, 01) is 01 which is decimal 1.
For BITAND(2,2) the binary value of AND(10, 10) is 10 which is decimal 2.
For BITAND(3,3) the binary value of AND(11, 11) is 11 which is decimal 3.

Following the same logic, the result for BITAND(4,4) is 4 and so on as shown 
in the table below.



Refer: Logic Gates (AND truth table)