Answer: Option (a) is the correct answer.
Step-by-step explanation:
Plasma is defined as an ionized gas that consists of positively charged ions and negatively charged electrons which are similar in number. In a plasma, molecules move rapidly due to high energy.
Therefore, when a gas is heated then it will gain more energy due to the phase transition. Hence, this phase transition will change gas into plasma.
Thus, we can conclude that as heat is added to a tube of gas, gas to plasma is the most likely phase transition to occur in the tube.