The correct answer is: Both statements are true.
An isoquant is a curve that shows all the combinations of inputs that produce the same level of output. The convexity of the isoquant curve is due to the diminishing marginal rate of substitution (MRS). The MRS is the rate at which one input can be substituted for another while keeping output constant. The MRS declines as the amount of one input increases, because the more of one input you have, the less valuable each additional unit becomes.
The complementarity of two inputs refers to the fact that they are used together in production and that the productivity of one input depends on the level of the other input. The greater the complementarity of two inputs, the steeper the isoquant curve will be. This is because if two inputs are highly complementary, then a small change in the amount of one input will require a large change in the amount of the other input in order to keep output constant.
Statement I is true because the isoquant curve is drawn convex to the origin due to the diminishing marginal rate of substitution.
Statement II is true because the lesser the convexity of the isoquant curve, the greater the possibility of the complementarity of the two inputs.