Plasma is

ANS:B - nothing but ionised gas.

Plasma is a state of matter that is often described as the fourth state of matter, distinct from solids, liquids, and gases. It consists of ionized gas particles containing positive ions and free electrons. Here's a breakdown of the given options:

1. A high temperature neutral gas: This is not entirely accurate. Plasma can be at high temperatures, but it is not neutral; it contains ions and free electrons.
2. Nothing but ionised gas: This statement is more accurate. Plasma is indeed an ionized gas where some or all of the atoms or molecules have lost or gained electrons, resulting in positive ions and free electrons.
3. A source of steady and highest controllable pressure: Plasma is not typically used as a source of pressure. It is more commonly used in applications such as lighting, plasma cutting, and fusion research.
4. Formed at very low temperature: Plasma is often formed at high temperatures, although the specific temperature range can vary depending on the application. Plasma can exist at both high and low temperatures, but it is generally associated with high-temperature environments such as those found in stars or during plasma cutting processes.