The correct answers are: My phone started ringing loudly while the professor was explaining the final assignment. I was desperately trying to silence the alarm when I dropped my heavy backpack. The whole class chuckled after I finally turned the device off.

While is used for continuous, ongoing background actions, so "while my phone started ringing" sounds unnatural because "started" is an instantaneous action.

The sentence "while I was desperately try" contains a grammar error; it must be "trying" to properly form the past continuous tense!