If Y Varies Directly as X, Find a Linear Function $y = 3.6$

Multiple Choice

If y varies directly as x, find a linear function which relates them.
- $y = 3.6$ when $x = 0.9$

A) $f ( x ) = 4 x$
B) $f ( x ) = 0.25 x$
C) $f ( x ) = x + 2.7$
D) $f ( x ) = 0.9 x$