Explanation:
We have t(x,y,z) as a scalar function. Divergence and curl are operations that can be done on a vector function. So they are not necessary for a scalar function. Grad is what is used for a scalar function.
Therefore the gradient is the sensible operator on temperature compared to others.
a. Grad T = δT/ δx.(xˆ ) + δT/ δy.(yˆ) + δT/ δz.(zˆ)
b. The gradient of T(x,y,z) gives a measure of the direction and the rate at which temperature changes in all directions.
C. The result is going to be a vector quantity.
I have written out the answer to a in an attachment.