Final answer:
The function that satisfies the given conditions is f(x) = ax² + b. We can choose values for a and b to meet the conditions.
Step-by-step explanation:
The function that satisfies the given conditions is option A. f(x) = ax² + b.
To be differentiable everywhere, a function must be continuous and have a derivative at each point. Option A represents a quadratic function, which is continuous and differentiable everywhere. So, we can choose values for a and b such that the function satisfies the conditions.
For example, let's say we choose a = 1 and b = 0. The function becomes f(x) = x², which is a quadratic function with positive value at x = 3 and a positive slope that decreases in magnitude with increasing x.