Answer:
A. 9
Explanation:
Triangle Inequality: the lengths of any two sides of a triangle add up to more than the third side.
B. 10 can't be the answer because the 4 cm and 6 cm sides would just meet at a point on the 10 cm sides.
C. 11 and D. 12 can't be answers because the 4 cm and 6 cm sides would not meet (kind of like windshield wipers that are too short).