Final answer:
Files that frequently change in size or content should be stored in the /var directory on a Linux filesystem. This directory is specifically designed for variable files, which are expected to change as the system operates.
Step-by-step explanation:
Which directory in a Linux file system should contain files that change often. The correct answer is a) /var. The directory /var stands for variable files, which is where the system stores files that are expected to change in size and content as the system is running. This includes things like logs, mail, and print queues, among others.
In contrast, /usr is generally for static files like binaries and libraries; /root is the home directory for the root user, and it's not a place for storing frequently changing files due to security reasons; /proc is a virtual filesystem that provides process and kernel information as files. Thus, /var is the appropriate choice for files that change often.