The equation of a line that is parallel to the x-axis is in the form \( y = k \), where \( k \) is a constant. In your options, the correct choice that represents a line parallel to the x-axis is:
\( y = 7 \)
The other options represent different types of lines:
- \( y = 7x \) is a line with a slope of 7 (not parallel to the x-axis).
- \( x = 7 \) is a vertical line (not parallel to the x-axis).
- \( x = 7y \) is also not parallel to the x-axis; it represents a relationship between \( x \) and \( y \) that can be rewritten as \( y = \frac{x}{7} \) (a diagonal line).
So, the correct answer is \( y = 7 \).