Answer:
D) 
Step-by-step explanation:
A linear relationship is of the form
, where,
is the slope and
is the y-intercept (constant).
For a proportional relationship, the value of
and thus it is of the form
Let us check each option and express it in the form above.
Option A:

This can be written as
. So,
.
Since, b \ne 0, therefore, it is not a proportional relationship.
Option B:

Here,
. So,
.
Since, b \ne 0, therefore, it is not a proportional relationship.
Option C:

Here,
. So,
.
Since, b \ne 0, therefore, it is not a proportional relationship.
Option D:

This can be rewritten as 
There is no y-intercept on this. So,
Since,
, therefore, it is a proportional relationship.
Therefore, the correct option is D.