If you are driving at the speed limit and another driver sounds his horn and starts to pass, you should -
a) slow down and allow him to pass
b) refuse to give way as he is already driving at the speed limit.
c) speed up to get out of his way.
d) sound your horn and wave to warn him

a. Slow down and allow him to pass

Explanation:

Since you are driving at the speed limit, you can not speed up to get out of his way. The other driver has more speed limit, and hence, he wants to pass you. If you slow down your speed he can pass and a possible accident could be avoided. Refusal to allow pass results, in most cases, a sense of jealous competition, which also could result in a fatal accident, or blowing to warn also could be irritating for another driver, who could take it as umbrage and will drive even faster in pursuit. A judicious decision is to slow down your car and allow him to pass.