Answer:
D, Examine his calculated Body Mass Index (BMI).
Step-by-step explanation:
A is irrelavant. His friends are not doctors and even doctors don't judge a persons health based on just appearance (at least not good ones).
B addresses the same as the first-- appearance-- which is counter productive as we sometimes see ourselves in a bad light and not reality.
C is not a good idea whatsoever. His friends could be taller than him, shorter than him, have more or less muscle than him, different medical needs, different activity levels, etc. All of these things are factors in what the ideal weight you "should" be. Someone who is shorter, is generally going to weigh less. This is NOT a good idea at all.
D, finally, is the right answer. Body Mass Index is the only correct answer here. While Body Mass Index still has flaws, this is what doctors and medical professionals use. This would be the most accurate based on a medical level.