Muscles convert chemical energy into
a. light energy.
b. nuclear energy.
c. mechanical energy.
d. electromagnetic energy.

Muscles convert chemical energy into MECHANICAL ENERGY.

Body muscles allow humans to consciously move different parts of their body. Muscles depend on the energy obtained from food to carry out their activities of movement. Energy that comes from food is called chemical energy. Mechanical energy on the other hand is the energy that is associated with one motion and position. Thus, in muscles, chemical energy is converted to mechanical energy.

practical process described pest part of scientific investigation.

because practical knowledge is powerful then therotical knowledge.

it can be observed without changing the identity of the object.

it depends on what substance you're talking about as well as many other factors. in quantum mechanics, the observation of particles affects the observers ability to measure certain properties. for example: you can know the velocity of a particle, but you can't know it's position; or you can know it's position, but not it's velocity. it's like trying to find out where a grain of sand is by throwing other grains of sand at it. once you find out where it is, it's been moved. it's also like trying to find an answer to the question of whether or not a tree falling in a forest makes a sound if nobody is there to hear it. paradoxically, the only way to find out is to be there and hear it.