The correct option is A 6.5 cm
Given, the lengths of two sides of a triangle are 5 cm and 1.5 cm, respectively.
Let sides AB=5 cm and CA=1.5 cm in ΔABC.
We know that sum of any two sides of a triangle is greater than the third side.
∴ BC+CA>AB and AB+CA>BC
⇒ BC+1.5>5 and 5+1.5>BC
⇒ BC>5−1.5 and 6.5>BC
⇒ BC>3.5 and BC<6.5
Hence, the length of the third side cannot be 6.5 cm.