What is the sign of the focal length for a convex lens as per the sign convention for spherical lenses?

The correct option is Positive.

According to the New Cartesian Sign Convention for spherical lenses, the rules for determining signs are as follows:
1. The optical center of the lens is taken as the origin, and the principal axis is taken as the x-axis.
2. The object is always placed to the left of the lens. This means that light travels from the left to the right side of the lens.
3. All distances measured in the direction of the incident light (to the right of the optical center) are taken as positive.
4. All distances measured in the direction opposite to the incident light (to the left of the optical center) are taken as negative.

For a convex lens, when parallel rays of light fall on the lens, they converge at the principal focus on the opposite side of the lens (the right side).
Since the principal focus of a convex lens lies on the right side of the optical center, the focal length (which is the distance from the optical center to the principal focus) is measured in the direction of the incident light.
Therefore, the focal length of a convex lens is always positive.