CAT 2020 Inequalities — Previous Year Question

Consider the four variables A, B, C and D and a function Z of these variables, $Z = 15A^2 - 3B^4 + C + 0.5D$. It is given that A, B, C and D must be non-negative integers and that all of the following relationships must hold:

i) $2A + B \le 2$

ii) $4A + 2B + C \le 12$

iii) $3A + 4B + D \le 15$

If Z needs to be maximised, then what value must D take?