Reciprocal levelling is a technique used in surveying to eliminate errors caused by curvature and refraction of the earth's surface. However, there is still a possibility of error that is caused by parallax.

Parallax error occurs when the line of sight of the telescope is not perpendicular to the staff being observed. This can happen when the observer does not have the eye exactly centered on the crosshairs of the telescope. As a result, the reading may be too high or too low.

Explanation of options:

A) Curvature: Curvature error occurs due to the shape of the earth. The surface of the earth is not flat, and therefore, it is not possible to achieve accurate measurements without accounting for this curvature. Reciprocal levelling is a technique that takes into account the curvature of the earth.

B) Refraction: Refraction occurs when light passes through different mediums and bends. This can cause errors in measurements, particularly in hot or cold weather conditions. Reciprocal levelling takes into account the effect of refraction by using a correction factor.

C) Non-adjustment of fine of collimation: The fine adjustment of collimation is critical to achieving accurate measurements. If the collimation is not adjusted correctly, it can lead to errors in the measurement. However, reciprocal levelling does not rely on the fine adjustment of collimation.

D) Parallax: Parallax error occurs when the line of sight of the telescope is not perpendicular to the staff being observed. This can happen when the observer does not have the eye exactly centered on the crosshairs of the telescope. As a result, the reading may be too high or too low. Reciprocal levelling cannot completely eliminate parallax error.

In conclusion, the correct answer is option D (parallax). While reciprocal levelling can eliminate errors caused by curvature and refraction, it cannot completely eliminate parallax error.