Consider L1={anbm|n=m}
and L2={anbm|n≠m}
then L1∩L2=ϕ
and L1∪L2=a∗b∗
Since there exits a DFA for L1∪L2
Hence option (b) is false,
Option (c) is also false since for both L1 and L2 there
Now, In order to verify option (a)
Consier L+1=a∗b∗
L2={anbm|n=m}
then, L1∩L2={anbm|n=m}
and L1∪L2=a∗b∗
Clearly, no DFA exists for L1∩L2
Here option (a) is also false.
Therefore, (d) is the only choice.