Minterm = product expression involving all the variable
Maxterm = Sum expression involving all the variable
Number of Minterm/Maxterms possible with n variables = $2^n$
In our case n = 3 which means number of minterms/maxterms = 8
For the given assignment, only way to get a minterm ie a product expression to evaluate as 1 is pq~r (where ~r denotes r complement)
And the remaining minterms ie 8-1=7 are all evaluated to be 0.
Similarly only maxterm to evaluate as 0 is ~p+~q+r and all remaining 7 are 1.
So,
A = D = 1
And B = C = 7