Eulers critical bulcking load

Question

For the case of a slender column of length $l$ , and flexural rigidity EI built-in at its base and free at
the top, the Euler’s critical buckling load is

(a)  $\frac{4\pi ^2EI}{l^2}$                            (b)$\frac{2\pi ^2EI}{l^2}$
(c) $\frac{\pi ^2EI}{l^2}$                               (d)$\frac{\pi ^2EI}{4l^2}$