Why does nuclear fusion occur in stars but not on Earth? A.
There are no elements on Earth that can undergo fusion.
B.
Too many free neutrons are present on Earth.
C.
Fusion reactions require a lot of heat and pressure.
D.
Elements capable of undergoing fusion can’t be enriched on Earth.

C. Fusion reactions require a lot of heat and pressure

Explanation:

A Nuclear fusion doesn't occur naturally on Earth because it requires temperatures far higher than Earth temperatures. Nuclear fusion takes place only at extremely high temperatures. That's because a great deal of energy is needed to overcome the force of repulsion between the positively charged nuclei.

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.