When using the sign convention for spherical mirrors, which direction along the principal axis is considered positive?

The correct option is: To the right of the origin (along + x-axis).

According to the New Cartesian Sign Convention used for spherical mirrors:
1. The pole of the mirror is taken as the origin (0, 0) of the coordinate system, and the principal axis is taken as the x-axis.
2. The object is always placed to the left of the mirror. This means that light travels from left to right.
3. All distances measured in the direction of the incident light (to the right of the origin or along the positive x-axis) are taken as positive.
4. All distances measured opposite to the direction of the incident light (to the left of the origin or along the negative x-axis) are taken as negative.
5. Distances measured perpendicular to and above the principal axis (along the positive y-axis) are taken as positive.
6. Distances measured perpendicular to and below the principal axis (along the negative y-axis) are taken as negative.

Therefore, along the principal axis, the direction to the right of the origin (along the positive x-axis) is considered positive.