Which of the following limits gives the slope of the tangent line to f(x) at x = a?
a) lim (x → a) (f(x + h) - f(x)) / h
b) lim (h → 0) (f(a + h) - f(a)) / h
c) lim (x → a) (f(x) - f(a)) / (x - a)
d) lim (h → 0) (f(x + h) - f(x)) / h