Given:
The equation for the direct variation is

To find:
The constant of variation k for the direct variation.
Solution:
If y is directly proportional to x, then

...(i)
Where, k is constant of variation or constant of proportionality.
We have,

It can be written as

Multiply both sides by -1.
...(ii)
On comparing (i) and (ii),we get

Therefore, the correct option is D.