The correct answer is B. Phosphorus.
Matchsticks are made up of a head, a stem, and a striking surface. The head of the matchstick is made up of a mixture of phosphorus sesquisulfide, potassium chlorate, and gum arabic. The stem of the matchstick is made up of wood. The striking surface is made up of red phosphorus.
When the matchstick is struck against the striking surface, the friction causes the red phosphorus to ignite. The heat from the ignition of the red phosphorus then causes the phosphorus sesquisulfide in the head of the matchstick to ignite, which in turn causes the wood of the matchstick to ignite.
Sulphur is not a component of matchsticks. Magnesium is a metal that is used in some types of matches, but it is not the main component of matchsticks. Potassium is a metal that is used in some types of matches, but it is not the main component of matchsticks.