What happens in a vacuum interrupter when the contacts separate?

In a vacuum interrupter, when the contacts begin to separate, an arc does indeed form as a result of the ionization of the medium between the contacts. This initial arc occurs because the electrical current tries to continue flowing even as the gap between the contacts is increasing. The gap becomes conductive due to the high voltage across it, which leads to the formation of an arc.

However, as the contacts continue to separate further, the conditions favor rapid cooling and the rapid reduction in pressure within the interrupter. This leads to the arc being extinguished almost immediately after formation.

While it is true that the quick response of the vacuum device allows for the rapid extinguishment of the arc, the defining characteristic at the moment when the contacts separate is the initial formation of the arc itself, which corresponds to the correct answer.