what phenomenon enables thermostats and thermometers to work?

Question

1-matter changes state whenever it's thermal energy increases
2-matter contracts as it is heated.
3-matter expands as it is heated
4-when heated, some metals expand whereas other metals contract

Answer 1

#3, your text book should have explained this to you.

Answer 2

3

Answer 3

I am going to go with number 3.