Very often, when a student submits mediocre work, it is because they have left out the details and examples that prove their point.
If I asked what you liked about the beach and you just said “sand,” I would walk away a bit bored and wanting to know more. The same is true of your writing. If the prompt poses a question, answer it and then explain the “why”. Give details and examples and use descriptive language whenever you can.
“I like the sand best because I have a shell collection, and whenever we go to the shore, I look for colorful additions to my set. I also enjoy building sand castles in the wet sand with my little sister.”
Now this is a full reason with detail and explanation. It also uses adjectives and descriptive language to let the reader form a picture and impression of his own.
Always go back and infuse your work with details and examples that bring your ideas to life!