Answer: a. -4°C.
b. He is incorrect because temperature is a vector quantity.
Explanation:
We know that the temperature is a vector quantity because it has both magnitude and direction.
The value of T represents the temperature that is 4 degrees C below zero is written as -4°C.
Also, if a student says that "T represents only positive numbers and zero, because there is no negative sign," then he is incorrect because temperature is a vector quantity (it can be negative).