1. Given situation

A source of light is kept at the focus of a convex lens.

2. Property of a convex lens

A convex lens has the property that if a point source is placed at its principal focus, then after refraction through the lens, the emerging rays become parallel to the principal axis.

3. Relation between rays and wavefront

Wavefront is always perpendicular to the direction of propagation of light rays.

• If rays are diverging from a point source, the wavefront is spherical.
• If rays are parallel, the wavefront is plane.

Since the emergent rays are parallel, the emergent wavefront must be a plane wavefront.

4. Checking options

• A: cylindrical $\to$ Incorrect
• B: spherical $\to$ Incorrect
• C: plane $\to$ Correct
• D: both spherical and cylindrical $\to$ Incorrect

5. Final answer

The shape of the wavefront is plane.