The correct answer is option 'D', which states that the horizontal distance between two parallel reinforcing bars should not be less than 25 mm. This requirement is based on the design principles and specifications for reinforced concrete (RC) beams.

Reinforcement in an RC beam is crucial for providing tensile strength and resisting bending moments. The main reinforcement bars are designed to carry the majority of the tensile forces in the beam. The coarse aggregate size used in the beam is 20 mm, which means that the maximum size of the aggregate particles is 20 mm.

To ensure proper bond between the concrete and the reinforcement bars, certain minimum clearances should be maintained. The clear cover is the distance between the outer surface of the concrete and the nearest surface of the reinforcement bar. The clear cover is necessary to protect the reinforcement from corrosion and to provide fire resistance.

The clear cover requirements are specified in design codes and standards. For example, as per the Indian Standard Code of Practice for Plain and Reinforced Concrete (IS 456:2000), the minimum clear cover for beams is specified as 25 mm. This means that the distance between the reinforcement bar and the edge of the beam should not be less than 25 mm.

In addition to the clear cover, there are also requirements for the spacing between parallel bars. The horizontal distance between two parallel reinforcing bars should not be less than the diameter of the bar. In this case, the main reinforcement consists of 16 mm bars, so the minimum spacing between them should be 16 mm.

Therefore, to satisfy both the clear cover requirement and the spacing requirement, the horizontal distance between two parallel reinforcing bars should not be less than the sum of the clear cover and the diameter of the bar, which is 25 mm + 16 mm = 41 mm. However, since the options provided do not include this value, the closest option is 25 mm, which is the correct answer.