Volume of model options: +,×,÷,-

option for volume of green model in model 2: 3, 12, 16, 60

•Volume of model 1:
16^3 [ + ] 20^3
because in order to get the entire volume you would need to combine the 2 separate parts.
•Volume of Green Model in Model 2:
it would be [ 12 ]
because if the models are equivalent in volume you would get the equation 16+20=24+x.
36=24+x Addition.
12=x= green model Subtraction.
Hope this helps and if needed, use as a reference in the future :)

Heptagon is the correct ans