Answer:
The correct answer is the last option: the assumption of the diminishing marginal productivity of each point.
Step-by-step explanation:
On the one hand, the isoquants are the curves used in the study of microeconomics that particulary show the different combinations of inputs that a company can use in the production of its product with the purpose of maximizing it.
On the other hand the law of diminishing marginal productivity establishes that as long as a particular factor in the production keeps increasing then the productivity of the produciton will slowly decrease with each increase of the factor mentioned before.
So in conclusion, the isoquants are convex curves because that law establishes that if a factor goes up, the productivity goes up as well but at a lower level until eventualy goes down.