The local linear approximation of a function f will always be greater than or equal to the function's value if, for all x in an interval containing the point of tangency,
$\displaystyle f''(x)<0$ means that the graph is always concave down. Looking at the graph will probably give you an idea why all linear approximations on such function are always larger or equal to the function.
I don't know. I gave you a graph to help you focus on a choice. Do you know what $\displaystyle f''(x)<0$ means? Do you know what a local linear approximation would look like? Did you completely understand the question?