Books are measured in kilograms (option 'C').

Books are measured in kilograms because weight is the most appropriate unit of measure for objects like books. The weight of a book can vary depending on factors such as the size, thickness, and number of pages. This makes kilograms a suitable unit of measure as it is commonly used to measure the weight of objects.

Here is a detailed explanation of why kilograms are the correct unit of measure for books:

1. Weight as a measure:
- Books are physical objects that have mass, which is a measure of the amount of matter in an object.
- Weight is a measure of the force exerted on an object due to gravity.
- Kilograms are the standard unit of measure for weight in the metric system.

2. Variations in book weight:
- Books can vary significantly in weight depending on various factors.
- Hardcover books are generally heavier than paperback books due to their thicker and more substantial covers.
- The number of pages in a book can also affect its weight. A thicker book with more pages will generally be heavier than a thinner book.
- The size of the book, such as its dimensions and thickness, can also contribute to its weight.

3. Common use of kilograms for measuring weight:
- Kilograms are widely used as a unit of measure for weight in everyday life and in various industries.
- Using kilograms as the unit of measure for books ensures consistency and compatibility with other weight measurements.

In conclusion, books are measured in kilograms because weight is the most appropriate unit of measure for objects like books. Kilograms are commonly used to measure the weight of objects, and books can vary in weight depending on factors such as size, thickness, and number of pages.

Books are typically measured in kilograms as they are not length or volume measurements.