On a system with separate partitions for /, /usr, /var & /tmp, which filesystem can safely be mounted read-only?

Answer: /usr

Explanation:
The /usr partition contains common executables and documents such as man pages (help files), all of which should not be changed by users. Therefore, this partition should be mounted as read only.

Reference: http://www.redhat.com/docs/manuals/linux/RHL-6.0-Manual/install-guide/manual/doc084.html