Final answer:
The line height property directly affects the vertical spacing in an HTML layout.
Step-by-step explanation:
The correct answer is C) Line height.
Line height refers to the vertical spacing between lines of text in an HTML layout. By adjusting the line height property in CSS, you can control the amount of space between lines of text. Increasing the line height will result in more vertical spacing, while decreasing it will reduce the spacing.
For example, if you have a paragraph with a line height of 1.5, the lines of text will be spaced out more compared to a paragraph with a line height of 1.0.