To answer the question on whether an equation represents a proportional relationship, we should understand what proportional relationships look like in mathematical form. A proportional relationship between two variables can be represented by an equation of the form:

$y = kx$

where $k$ is a constant of proportionality, and $x$ and $y$ vary together such that $y$ is always a constant multiple of $x$.

Looking at the answer choices:

1. $y = k + x$ — This is not a proportional relationship because it adds a constant $k$ instead of multiplying $x$ by $k$.
2. $y = k$ — This represents a constant relationship, not a proportional one between $y$ and $x$.
3. $y = kx$ — This matches the form of a proportional relationship.

So, the correct choice is $y = kx$.

Would you like more details on proportional relationships or further clarification on this topic?

Here are some related questions to expand your understanding:

1. What is the difference between a proportional and a non-proportional relationship?
2. How do you find the constant of proportionality $k$ from a table of values?
3. Can you graph a proportional relationship, and how would the graph look?
4. Why does $y = k + x$ not represent a proportional relationship?
5. How does the concept of proportionality apply in real-life situations?

Tip: When identifying proportional relationships, check if the graph passes through the origin (0,0) and if there’s a constant ratio between $y$ and $x$.