Since
![\sqrt[3]{32}\not\in\mathbb Z](https://tex.z-dn.net/?f=%5Csqrt%5B3%5D%7B32%7D%5Cnot%5Cin%5Cmathbb%20Z)
, you can omit B and C as possible solutions.
Now,

but since 16 is not divisible by 3, A is not correct either.
So D is the answer. To verify:
6 groups of five people
because 30/5 = 6
so 6 groups
No triangles can be constructed because
a+b>c
4+5 is not greater than 9