Answer:
Option D
Explanation:
A relation is said to be a function in which for a distinct input there is a single output.
If the input pairs with multiple outputs then the relation is not a function.
We can observe each option one by one.
In case of option A, the inputs are not repeated, there is only one output for each input.
In case of option B, the property of one output to each input doesn't hold as 3 has two outputs.
In Option C, -5 and -10 both are repeated.
In option D, 20 has two outputs which fails option D to be categorized as a function.
So option A is the correct answer..