Question 20: C. much
Correct answer: C. much(Water is an uncountable noun, so "much" is the appropriate choice.)
 
Question 21: A. a variety
Correct answer: A. a variety (The phrase "a variety of" is commonly used to refer to different types of something, which fits the context of proteins, carbohydrates, and fats.)
 
Question 22:A. eight
Correct answer: A. eight (The word "eight" is used correctly here without an article because we are talking about a quantity of hours.)
 
# Sentence Arrangement
 
Question 23:A. a-b-c-d
Correct answer: A. a-b-c-d (This arrangement makes a logical flow about adopting healthy habits: stating the importance first, followed by benefits of exercise, sleep, and stress management.)
 
Question 24: A. a-c-d-b
Correct answer: A. a-c-d-b (This arrangement starts with a greeting, followed by expressing a desire for advice, stating the current situation at school, and finally mentioning the problem with parents.) 
 
So the completed passage would look like this:
 
Dear Students,  
To improve your overall health, consider the following recommendations:  
Stay Hydrated: Drink much water throughout the day to keep your body hydrated.  
Balanced Meals: Make sure to eat a variety of proteins, carbohydrates, and fats in your meals.  
Sleep Well: Aim for eight hours of sleep each night to help your body recover.  
Best regards,  
[Nick]