Step 1: Understanding socially necessary labour time.
Karl Marx defined socially necessary labour time as the labour time required to produce a commodity under normal conditions of production, with average skill and intensity prevailing in a given society.
Step 2: Role of historical and social conditions.
Marx emphasised that labour time is not absolute but socially determined. It varies according to historical context, technological development, and social organisation, which differ across time and place.
Step 3: Elimination of incorrect options.
Annual rainfall and natural resource output may affect production indirectly but do not define socially necessary labour time.
Average productivity affects labour time inversely, not directly.
Step 4: Conclusion.
Therefore, socially necessary labour time directly varies with the specific time period and place in which production occurs.