Core-concept association: identify the option most fundamentally connected to the natural phenomenon 'lightning' (distinguish cause, effect, and co-occurring events)

Given data

• Target word: lightning.
• Options: electricity, thunder, brightness, rain, wind.

Concept/Approach (why this method)

Identify the physical nature of lightning versus its effects or common companions. Lightning is an electrical discharge; thunder is sound produced by the rapid expansion of air due to lightning's heat.

Step-by-Step reasoning
1) Lightning = a massive electrical discharge between clouds and ground/clouds.2) Thunder is a consequence (acoustic effect) of lightning, not its essence.3) Brightness describes an appearance, not the core nature.4) Rain and wind may accompany storms but are not required for lightning to occur.5) Thus the fundamental link is electricity.

Verification/Alternative

Lightning can occur in dry thunderstorms (little rain) and is still electrical; thunder always follows lightning, confirming electricity as the root.

Common pitfalls

• Picking thunder due to frequent pairing (thunder & lightning) rather than causal substance.

Final Answer
electricity