The correct answer is D. "Let's go to the gym together, you pick the activity."
Explanation
A positive influence is the ability of a person or group of people to influence the attitude, thinking, actions, speech, expression, and others of another person in a positive way. A positive influence as the name implies means something favorable or beneficial for a person, in this case, a positive influence to make the influenced individual get good things from that influence. On the other hand, there is a negative influence, which is quite the opposite, that is to say, that it does not bring benefits for the person but damage, unfavorability and bad things. According to this, the statement that shows a positive influence is "Let's go to the gym together, you pick the activity" because, in this case, one of the friends of Mike is inviting him to do exercise in the gym which is positive for Mike's health. So, the correct answer is D. "Let's go to the gym together, you pick the activity."