What is the purpose of the mantra during meditation?
a.) it allows you to concentrate on one peaceful sound
b.) it lets everyone know you're meditating
c.) to release endorphins

A.      It allows you to concentrate on one peaceful sound

 Mantra is a Sanskrit word which literally translates to “instrument of the mind.” From scattered thought and outward focus, mantras lead the mind to stillness and inner focus. Concentrating on one thought or on a phrase that doesn’t evoke strong emotions is a great start to achieve inner peace. Repeating the mantra will then prevent distracting thoughts and ideas from popping up. The repetition will gradually slip the mind into a state of thoughtlessness which is better than forcing the mind suddenly to do so.

Well if you pick a machine to work on and that's gonna be your exercise if you don't stick to that machine your muscle strength your muscle mass rather will change because you haven't been doing it you change something in your exercise routine which changes your body's form of dealing with things for example you might become weaker or stronger depending on what kind of exercises you do in whether or not the machine you picked actually your muscles grow.