Answer:
It could be any of them.
Explanation:
First off, It could be said that (2c-d)(2c-d) doesn't belong because it includes two subtractions signs and none of the others do.
Secondly, It could be said that (2c+d)(2c-d) doesn't belong because it includes one subtraction sign and one addition sign and none of the others do.
Thirdly, It could be said that (2c+d)(2c+d) doesn't belong because it includes two addition signs, and none of the others do.
Finally, it could be said that (c+d)(c+d) doesn't belong because it doesn't have a multiplying factor of 2 within the parentheses.
I hope this helps :)