Rosalyn's arms felt tired and fatigued after she held her cousin's baby for a few minutes. what area of fitness does she need to improve?

I found the missing choices, these are: 
Cardiorespiratory endurance
Body composition
Flexibility
Muscular strength

Rosalyn's area of fitness that needs to improve is MUSCULAR STRENGTH. 

Muscular strength is defined as the amount of force a muscle can produce with a single maximal effort. In Rosalyn's case, she needs to develop her arm muscles to enable her to hold her cousin's baby without feeling any fatigue.

Aproximately 5 pieces can be cut from it.

Answerscientists are able to work both with current and future technology;