Which unit of measurement is commonly used for font sizes in CSS?

The correct answer is that all of the mentioned units can be used for font sizes in CSS, making it comprehensive to say that they all apply.

Pixels (px) are a standard unit of measurement in CSS that provides a fixed size for fonts. This unit is often used when precision is needed in layout design, allowing developers to specify exact font sizes.

Percentage (%) is also widely utilized for font sizes, as it allows for responsive design. When font sizes are defined using percentages, they are relative to the parent element's font size, providing flexibility that can adapt based on the context in which it is rendered. This is particularly useful for ensuring that text scales appropriately across different devices and screen sizes.

Em units are another relative sizing mechanism that changes font size based on the font size of its parent element. This allows for a cascading effect where changes to the parent size can propagate through all child elements that use em units. Using ems provides a scalable way to manage text throughout a website since it maintains proportional relationships.

Since each of these units serves a purpose in web design and offers different advantages—like precision, responsiveness, and scalability—it's accurate to conclude that they are all commonly used for font sizes in CSS.