Which of the following can cause a stroke?

Question

$\begin{array}{1 1} (a)\; \text{A blood clot in an artery in the brain} \\ (b)\; \text{A blood clot in an artery in the heart} \\ (c)\; \text{A blood clot in an artery in the leg} \\ (d)\; \text{A blood clot in an artery in the lungs} \end{array}$