Thank you for the opportunity to help you with your question!
Hey! The most common rule for forming plural nouns is C. Adding -s to the word. This is just a basic rule that you can memorize, and once you start seeing plural nouns more often, you'll recognize when to use it.
Some examples: nouns, starts, blankets, spoons...
Please let me know if you need any clarification. I'm always happy to answer your questions.