Answer: negation.
Step-by-step explanation: In a triangle, all angles cannot be right angles, because if it is so, then 90°+90°+90°=270°>180°. Hence, not possible. Also, a triangle has only three sides, so the sentence that a triangle has four sides is also not correct.
So, the given statement is a combination of two negative statements, and so, it is negation.
Thus, the correct answer is negation.