Which of the following is not part of necessary outdoor preparations.? a: tell someone where you’re going. b: make sure your cell phone is charged. c: check the weather forecast. d: pack enough food and water.

I would say B.
A is important because your friends and family may report you missing.
C is important because you don't want to be caught outside in a storm.
D is important for obvious reasons. 

You don't necessarily need your cell phone and the question is asking what is NOT necessary.