The axons of a motor neuron release a neurotransmitter which generates an impulse to contract a muscle. what is this neurotransmitter called? acetylcholine

   Acetylcholine

Acetylcholine is a neurotransmitter, which is a chemical released by the axons of a motor neuron which causes muscles to contract. It has other functions: activates pain responses and regulates endocrine and REM sleep functions. Deficiencies in acetylcholine cause muscle weakness.

The answer is Acetylcholine. Acetycholine is a chemical messenger, a neurotransmitter that is released by nerve cells in many parts of the peripheral nervous system. It controls the contraction of skeletal muscles or voluntary muscles and also affects the contraction of smooth and cardiac muscle. It binds to the nicotinic ion channel receptors on the muscle cell membrane, causing the ion channels to open. Then calcium ions flows into the muscle cel, initiating a sequence of steps that finally produce muscle contraction.

only b 

Mitochondria produce atp, a high energy compound. atp is used for all sorts of enzyme-mediated reactions, such as making proteins/nucleotides/etc, certain compounds cross the cell membrane, muscles move, etc.  without this source of energy, a cell will die. it will not be able to repair itself, defend against pathogens, etc.