Statement I is true (if a function's derivative exists at x = a, then the function is continuous there).
Statement II is false. (A point of inflection is a where the graph of f changes concavity, so the 2nd derivative will change signs there (and often will = 0), but f(a) ≠ 0 necessarily.)
Statement III is false. If f continuous, then ∫abf(x)dx = F(b) - F(a) , where F'(x) = f(x).