Concept:Backing up extension messages means helping farmers act on the advice they receive.
Explanation:Extension agents give farmers messages about improved farming practices.
For those messages to be useful, farmers must have access to the inputs the agent recommends.
An agent backs up the message by directing farmers to where they can find such farm inputs.
This closes the gap between advice and real action on the farm.
Other options like providing marketing channels or machines are not direct backup roles of an extension agent.
Only option B shows the agent supporting the message with a practical follow-up service.
Answer:B. assist farmers in locating sources of farm inputs.