Consider the four variables A, B, C and D and a function Z of these variables, Z = 15A² − 3B⁴ + 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 ≤ 2 ii) 4A + 2B + C ≤ 12 iii) 3A + 4B + D ≤ 15 If Z needs to be maximised, then what value must D take?