The correct answer is: C. toe of the switch to the theoretical nose of crossing.
The lead of crossing is the distance from the toe of the switch to the theoretical nose of the crossing. The theoretical nose of the crossing is the point where the theoretical extension of the switch tongue would intersect the theoretical extension of the crossing frog.
The heel of the switch is the point where the switch tongue meets the switch rail. The toe of the switch is the point where the switch tongue meets the frog. The frog is the part of the crossing that allows the switch tongue to move from one rail to the other.
The lead of crossing is important because it determines the amount of space that is available for a train to cross the switch. If the lead of crossing is too short, the train may not be able to cross the switch without derailing.