Answer:
The constant of proportionality is option D i.e 5.
Step-by-step explanation:
Variation:
Variation problems involve fairly simple relationships or formulas, involving one variable being equal to one term. There are two types of variation i.e.
- Direct variation
- Inverse variation
Direct Variation:
Mathematical relationship between two variables that can be expressed by an equation in which one variable is equal to a constant times the other.
Example 
where, k is constant of proportionality.
The above given example is of Direct Variation
∴ y = 5 x
∴ k = 5 = constant of proportionality.
Inverse Variation:
Mathematical relationship between two variables which can be expressed by an equation in which the product of two variables is equal to a constant.
Example 
where, k is constant of proportionality.