Here it is a priority encoder as input bits are $2^n$ and output bits are n
also priority is defined as D3 >D2>D1>D0
i.e. when $D3$ =1 output is $0 1$ we don’t care about others, as others have less priority than $D3$
similarly for other if $D3$ =0 $D2$=1 output is $10$ we don’t care about others and so on