Address-decoder and its address-range
All chips have active low chip-enable and a glue logic in the form of a gate, driven by A5 and A6 bits of the 6 bit address bus [A5 A4 A3 A2 A1 A0] . So A5 and A6 bits decide which chip will represent what address range.


MA is enabled only when A5 = 1 and A4 = 1. All other chips are disabled and hence only MA drives the data bus. So it will address the memory from  110000 to 111111.
Similarly MB is enabled only when A5 = 1 and A4 = 0. So it will address the memory from  100000 to 101111.
MC: 010000  to 011111.
MD: 000000 to 001111.