Correct interpretation using adjective clause

I sometimes wonder which of the following ( a) or b) ) should be the correct interpretation for the example below:

Example: The figure illustrating the example of A.

a) The figure which illustrates the example of A.
b) The figure which is illustrating the example of A.

This is just one example using the "ing" form in an adjective clause, but I would like to know whether the answer must be always either the case a) or b), or whether both are considered to be correct.