The output of a 2-input multiplexer is connected back to one of its inputs as shown
in the figure.

Match the functional equivalence of this circuit to one of the following options. 

A.

D Flip-flop

B.

D Latch

C.

Half-adder

D.

Demultiplexer

Solution:

Qn+1 = SQn + SQn

Qn+1 =Qn(S + S)

Qn+1 =Qn

Qn+1 is generated after delay. In a flip-flop, we always consider the edge, but the above circuit is level-sensitive, therefore, it is equivalent to a D-latch.