The alpha particles leave visible tracks in the cloud chamber because

ionization by the alpha particle seeds the formation of clouds where the particle has passed.
the kinetic energy of the alpha particles heats the air.
the alpha particle is so energetic that it leaves a smoke trail behind.
the alpha particle strikes nitrogen molecules and breaks them into atomic nitrogen.

The inner atmosphere of a cloud chamber is composed of an easily ionizable gas, this means that little energy is required to extract an electron from an atom. This gas is maintained in the supercooling state, so that a minimum disturbance is enough to condense it in the same way as the water is frozen.  

This is how alpha particles are able to ionize some atoms of the gas contained inside the chamber when they cross the cloud chamber.

These ionized atoms increase the surface tension of the gas around it allowing it to immediately congregate and condense, making it easily distinguishable inside the chamber like a small cloud. In this way, it is perfectly observable the path the individual particles have traveled, simply by observing the cloud traces left in the condensed gas.  

the correct choice is a .

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