Minimum spacing between horizontal parallel reinforcement of different sizes is an important consideration in construction to ensure the structural integrity and durability of the reinforced concrete element. The correct answer to this question is option 'B', which states that the minimum spacing should be one diameter of the thicker bar.

Explanation:

Importance of spacing:
- The spacing between parallel reinforcement bars affects the bond strength between the bars and the surrounding concrete.
- It also influences the crack width control and the ability of the reinforcement to resist tensile forces.
- Insufficient spacing can result in congestion of reinforcement, making it difficult to achieve proper compaction and cover concrete, which can lead to poor concrete quality and durability.

Consideration of different bar sizes:
- In reinforced concrete structures, different sizes of reinforcement bars are used depending on the design requirements and the loads to be carried.
- The diameter of the reinforcement bars affects their strength, ability to resist bending, and bond with concrete.
- When different sizes of reinforcement bars are used in the same element, it is necessary to determine the minimum spacing between them to ensure proper functioning of the reinforcement.

Reason for choosing option 'B':
- Option 'B' states that the minimum spacing should be one diameter of the thicker bar.
- This choice is based on the principle that the larger size reinforcement bars require more space for proper compaction and to provide sufficient cover concrete.
- By providing a minimum spacing equal to the diameter of the thicker bar, the congestion of reinforcement can be avoided and the desired concrete quality can be achieved.
- This spacing also ensures that the larger bars have enough room to develop their full tensile strength and bond with the surrounding concrete.

Alternative choices:
- Option 'A' suggests that the minimum spacing should be one diameter of the thinner bar. This choice may lead to insufficient spacing for the thicker bar, resulting in congestion and compromised concrete quality.
- Option 'C' suggests that the minimum spacing should be the sum of the diameter of the thinner bar and the thicker bar. This choice may result in excessive spacing, leading to unnecessary use of reinforcement and increased construction costs.
- Option 'D' suggests that the minimum spacing should be twice the diameter of the thinner bar. This choice may not provide enough space for the larger bar, potentially compromising its performance and bond strength.

Conclusion:
- The correct choice for the minimum spacing between horizontal parallel reinforcement of different sizes is option 'B', which states that the spacing should be equal to one diameter of the thicker bar.
- This choice ensures proper compaction, sufficient cover concrete, and optimal bond strength, leading to a durable and structurally sound reinforced concrete element.