Avoltmeter is a device that measures

A voltmeter its instrument used for measuring electrical potential difference between two points in an electric circuit. An ammeter is a measuring device used tomeasure the electric current in a circuit.

It measures the potential difference (voltage)

The answer is b.the gaseous state is electrically positive, while the plasma state is electrically negative. 

Texture and composition are different because they are not the same aspect. texture has to do with feel and so on, where as composition is the chemical makeup which you cant touch like you can texture.