In mechanical engineering, a pair refers to the connection between two elements that allows relative motion between them. A higher pair is a type of pair that has a greater number of degrees of freedom and allows both linear and rotational motion.

Let's analyze each option to determine which one is a higher pair:

a) Belt and pulley:
A belt and pulley system is a type of mechanical power transmission device. It consists of a flexible belt and one or more pulleys, which are wheels with a grooved rim designed to support and guide the belt. The belt transfers rotational motion from one pulley to another. This system allows both rotational and linear motion, as the belt can move along the pulley's circumference while the pulley itself can rotate. Therefore, a belt and pulley system is considered a higher pair.

b) Turning pair:
A turning pair, also known as a revolute pair, is a type of pair that allows only rotational motion between two elements. It consists of a cylindrical or spherical surface in contact with another surface, enabling the relative rotation between them. A turning pair does not allow linear motion, only pure rotational motion. Therefore, it is not a higher pair.

c) Screw pair:
A screw pair is another type of pair that allows both linear and rotational motion. It consists of a screw thread and a nut or a screw thread and a screw hole. The rotation of the screw causes linear motion of the nut or screw hole, allowing both types of motion. Therefore, a screw pair is considered a higher pair.

d) Sliding pair:
A sliding pair, also known as a prismatic pair, allows only linear motion between two elements. It consists of a sliding contact between two surfaces, enabling relative motion in one direction only. A sliding pair does not allow rotational motion, only pure linear motion. Therefore, it is not a higher pair.

Based on the analysis above, the correct answer is option 'a) Belt and pulley' as it is a higher pair that allows both linear and rotational motion.