The correct answer is: C. Verbally
A walkie-talkie is a handheld device that allows two-way radio communication over short distances. A written memo is a formal document that is used to communicate information in writing. Verbal communication is the act of speaking to someone.
In the context of the question, if the warner signal of any station becomes defective in off condition, a verbal message is issued to the loco pilot from the station in rear. This is done to ensure that the loco pilot is aware of the defect and can take appropriate action.
The other options are incorrect because they are not the most effective way to communicate this information to the loco pilot. A walkie-talkie could be used, but it is not as reliable as a verbal message. A written memo could be used, but it would take longer to deliver and the loco pilot may not have time to read it.