Answer now. "correctly."

what is the purpose of a persuasive letter?

to demonstrate the steps in a process
to inform readers about meaningful research
to convince readers to take action or support a cause
to explain a societal problem that needs to be addressed

To convince readers to take action or support a cause

Explanation: The verb to persuade (from which the adjective persuasive comes) means to cause someone to do a specific action or believe in a certain idea through the use of reasoning, presenting logical arguments; demonstration, explanation and information may be part of the persuasive strategy, but those are not the main purpose of a persuasive letter.

The confounding variable is the Psychology graduate

The control is the food I think

Explanation:

Without the graduate there would be no other variable but with the graduate they are able to have the control and charts