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Given:

4 relations in the options.

To find:

The relation which represents a function.

Solution:

We know that, a relation is a function if there exist a unique output value for each input value.

In option A, we have two output values y=-5 and y=1 for single input value x=-1. So, it is not a function.

In option B, we have two output values y=2 and y=4 for single input value x=3. So, it is not a function.

In option C, we have two output values y=1 and y=7 for single input value x=-1. So, it is not a function.

In option D, we have unique output value for each input value. So, it is a function.

Therefore, the correct option is D.