Questions

Rust forms when oxygen reacts with chemicals in rocks, and that is often the case with many other elements in general. Like a copper nail being exposed to the air will rust.

Nucleus serves as the control center for cell metabolism and reproduction.

"The moon smiled at me" is the example of personification in the given options.

Personification is a technique in which we give objects human traits like emotions, gestures, speech,... by using metaphor. 

Here moon is an object which cannot smile but using metaphor sense, we gave that sense to it.

The one word substitution for Of outstanding significance is monumental which has a meaning of great in importance, in size or any such characteristic.