The correct answers are The new waiter was in a hurry and dropped the soup. and I am at a loss to explain why the oven exploded.

In English, certain fixed expressions require the indefinite article "a" or "an" and cannot be changed or omitted.

• "In a hurry" is the correct fixed expression (not "in hurry").

• "At a loss" is correct (meaning confused or not knowing what to do).

• "Make a fuss" requires the article (so "make fuss" is incorrect).