Q.Assertion: The environmental conditions of the tropics are favourable for speciation and diversity of organisms. Reason: The climate, seasons, temperature, humidity and photoperiod are more or less stable and congenial.

Step 1. Check the Assertion: the chapter states directly, 'Environmental conditions of the tropics are favourable not only for speciation but also for supporting both variety and number of organisms' — so the Assertion is true.

Step 2. Check the Reason: the chapter's list of reasons for tropical richness includes, 'Climate, seasons, temperature, humidity, photoperiods are more or less stable and encourage both variety and numbers' — so the Reason is also true, word-for-word supported by the text. …

Verify the Assertion and Reason separately against the chapter's 'reasons for the richness of biodiversity in the Tropics' list, then confirm the Reason is presented as a …

• Marking the Reason as true but 'not the correct explanation' without checking that the chapter itself lists climate stability as one of the direct reasons for tropical richness. …