What a fool you are!

… only one choice is an outline.

How is this difficult, other than if you are line blind?

The correct choice is A.

If you pick B, A is also red, and C is also a filled solid. So B is not unlike either A or C.

If you pick C, A is also an irregular 4-gon, and B is also filled solid with color. So C is not unlike either A or B.

But if you pick A, while A does have elements in common with B and C…

(it shares ‘red’ with B, and ‘4-gon’ with C)

… it is also unlike each of them singly, as well as both of them together, in that it is a triangle.

A is the only choice where ‘is unlike the other two’… is true, in any sense.

It has a distinct property, not found in any member of the remainder set, nor shared by the remainder set as a group.