Final answer:
A linear equation with a positive slope is represented on a coordinate plane by a straight line that increases from left to right. This line's steepness correlates with the value of its slope, where a larger slope signifies a steeper ascent.
Step-by-step explanation:
A linear equation with a positive slope on a coordinate plane will look like a straight line increasing from left to right. Neither a vertical nor a horizontal line correctly represents a line with a positive slope. In particular, let's refer to Figure A1 which shows Slope and the Algebra of Straight Lines. This figure has x on the horizontal axis and y on the vertical axis, with the slope of the line being 3. This means there is a rise of 3 on the vertical axis for every increase of 1 on the horizontal axis, indicating that the line slants upwards as you move from left to right across the graph. The steepness of the line depends on the value of the slope. The higher the positive value of the slope, the steeper the line will go upward. This corresponds to option C, a straight line increasing left to right, correctly answering the question.