A straight line parallel to the y-axis has equation ______.

Options

A. x = a
B. y = a
C. y = x
D. `y=-x`

Answer

To solve the question, we need to determine the equation of a straight line that is parallel to the y-axis.

### Step-by-Step Solution:

1. **Understanding the Characteristics of a Line Parallel to the Y-Axis**:
A line that is parallel to the y-axis does not change its x-coordinate, meaning that for any point on this line, the x-coordinate remains constant.

2. **Formulating the Equation**:
If a line is parallel to the y-axis, it can be represented by the equation:
\[ x = a \]
where \(a\) is a constant. This means that for every point on this line, the x-coordinate is equal to \(a\) while the y-coordinate can take any value.

3. **Conclusion**:
Therefore, the equation of a straight line parallel to the y-axis is of the form:
\[ x = a \]
where \(a\) is a specific value.

### Final Answer:
A straight line parallel to the y-axis has the equation \(x = a\).