Carla needs more sleep before she takes the exam."
By regularly practicing exam-style questions, Carla can familiarize herself with the format, question types, and time constraints, which will boost her confidence and performance on the actual exam.
Sleep" is an uncountable noun, and when talking about needing more of it, you don't use the article "a" before it. The definite article "the" is used before "exam" because it refers to a specific exam that Carla is going to take.
So, the complete and grammatically correct sentence is: "Carla needs more sleep before she takes the exam."
See text
Carla needs more ___ before she takes ---exam.
O of the sleep, the
sleep, a
O of the sleep, a
O sleep, the