In B, the magnitude of a number does not depend on its sign. Clearly, for B, if x is 2, and y is -6, the expression is not true.
C essentially boils down to this, if you add 5 to both sides:
$$\frac{x}{3} > \frac{y}{3}$$
Multiply both sides by 3:
$$x > y$$
If x is a positive number and y is a negative number (as stated by the question), this is always true! Since, well, a positive number is always greater than a negative number. (Wait) (Wondering)
Therefore, C is the right answer.