Final answer:
The manual shut-off valve should be installed so that it is accessible without requiring tools to ensure quick and easy operation in an emergency.
Step-by-step explanation:
The student's question pertains to the installation requirements of a manual shut-off valve. In most jurisdictions, such a valve should be installed by specific safety standards and building codes. The valve is required to be accessible without requiring tools so that in the event of an emergency, it can be quickly shut off. The correct answer to the question is: a) Accessible without requiring tools. The other options may not comply with safety standards, as valves should not typically be concealed or located in places that are difficult to access during an emergency.