The frequency of direct circuit is

a) 100Hz b)greater than AC

c) less than AC d) 0 Hz

please explain how you always get the answers to this kind of question like the general way of knowing

Direct current does not alternate so there are no cycles of amplitude (Hz). 1 hertz is 1 cycle per second, standard AC (alternating current) in the states is 60Hz. So that means in one second the fluctuation of the sine wave goes back and forth 60 times. Direct current is just that, direct. Its just a straight line, however with the use of a 555 timer and some other components you are able to create a frequency with DC. It is more of a pulse though and not an actual sine wave.

Oh so the answer is D.

By definition the frequency of DC is zero.

d.