How to find a parent with a known class in jQuery?

Pass a selector to the jQuery parents function:

d.parents('.a').attr('id')

EDIT Hmm, actually Slaks's answer is superior if you only want the closest ancestor that matches your selector.

Assuming that this is .d, you can write

$(this).closest('.a');

The closest method returns the innermost parent of your element that matches the selector.

You can use parents() to get all parents with the given selector.

Description: Get the ancestors of each element in the current set of matched elements, optionally filtered by a selector.

But parent() will get just the first parent of the element.

Description: Get the parent of each element in the current set of matched elements, optionally filtered by a selector.

jQuery parent() vs. parents()

And there is .parentsUntil() which I think will be the best.

Description: Get the ancestors of each element in the current set of matched elements, up to but not including the element matched by the selector.