I think I figured it out.

The clique number is the maximum amount of vertices that are all adjacent to one another.

ABD = three vertices all adjacent to one another.

ACD = three vertices all adjacent to one another.

Since the maximum amount of vertices adjacent to one another is 3. The clique number is 3.

ABCD is not a clique because BC aren't adjacent. Also if the edge between A nd D did not exist the click number would be 2 because AB BD CD AC would be the maximum amount of vertices adjacent to one anohter.

I hope this helps someone some day having trouble figuring this out.