In the case of a continuous RC (reinforced concrete) beam, the live load should be placed on the span plus alternate spans in order to obtain the maximum positive span moment. This means that the live load should be applied on the span under consideration, as well as on the adjacent spans.

Explanation:
1. Understanding Continuous RC Beams:
- A continuous RC beam is a type of beam that is supported by multiple supports along its length, creating a continuous load path.
- The supports in a continuous beam are typically designed to be flexible and allow for some rotation, which results in a redistribution of moments along the beam.
- The bending moments in a continuous beam are influenced by both the dead load (permanent weight of the structure) and the live load (temporary loads such as people, furniture, vehicles, etc.).

2. Maximum Span Moment:
- The maximum positive span moment occurs at the midspan of a simply supported beam under a uniformly distributed load. However, in the case of a continuous beam, the moment distribution is different.
- Due to the redistribution of moments in a continuous beam, the maximum positive span moment may not necessarily occur at the midspan.
- The live load placement plays a crucial role in determining the maximum positive span moment in a continuous beam.

3. Live Load Placement for Maximum Positive Span Moment:
- Placing the live load on all the spans or on alternate spans starting from the left (options a and b) may result in a different distribution of moments along the beam, but it may not necessarily lead to the maximum positive span moment.
- Placing the live load only on spans adjacent to the span under consideration (option c) would not fully capture the effect of the live load on the entire beam, potentially leading to an inaccurate estimation of the maximum positive span moment.
- The correct option, option d, suggests placing the live load on the span under consideration as well as on alternate spans. This approach ensures that the live load is distributed across the entire beam length and allows for an accurate estimation of the maximum positive span moment.

In conclusion, in order to obtain the maximum positive span moment in a continuous RC beam, the live load should be placed on the span plus alternate spans. This approach ensures a more accurate estimation of the moment distribution and allows for an appropriate design of the beam.