What is the circuit's logic diagram of a (2-bit binary to decimal) encoder
it depends which 2 bits you choose out of 4 because you can only get 4 out of 10 decimal equivalent single outputs.


for a simple reduction from schematic:
with a pencil or your mind, tick off all the first 4 outputs back to the wired inputs and delete the rest not used.
it should only require 2 inputs as given in your question
or easier trace the 1st two bit inputs A,B to the 4 essential  outputs 2^2=4 leaving C,D=0


Since the NAND gates are redundant and two inversions cancel we get.