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

B. Make sure your cell phone is charged. You do NOT need your cellphone to be outside at all. So having it charged to go out has no reason to. I hope this helped! :)

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. 

Make sure your cell phone is charged

Explanation:

A cell phone is not needed outside