Explanation:

Assertion (A) is true: The Congress leaders boycotted the Simon Commission because it was an all-white commission appointed to review constitutional reforms in India, with no Indian representation. Reason (R) is also true: Lala Lajpat Rai died from injuries sustained during a police lathi charge while protesting against the Simon Commission. However, (R) is not the correct explanation of (A). The boycott (A) was decided due to the exclusion of Indians from the commission, while Lala Lajpat Rai's death (R) was a tragic consequence of the protests against the commission, intensifying opposition but not being the initial reason for the boycott itself.