The correct answer is: C) You should eat corn at every meal.
An explicit message is characterized by the direct way it tells things. It leaves no room for vagueness, implication or ambiguity, going straight to the main point. When receiving an explicit message no questions are left, there are not hidden meaning or intents.
The sentece "You should eat corn at every meal.", has those characteristics, it tells you what to do, nothing more, nothing less.
The other sentences try to deliver the same message but in a vague way, they relate eating corn with possitive things like patriotism. This is more related to subliminal messages in which things are not easily perceived, the real meaning is hidden. As a result a person can do something based on the message, believing that his actions have one particular cause (e.g. eating corn to be patriotic) but actually they respond to unknown causes (e.g. the corn industry).
Solid in this context means reliable and trustworthy
