The sentence in question requires words that logically and grammatically fit. When we break down the sentence, we see it discusses a significant realization of a generation and the power of change through personal mindset. Let's analyze the options:
- gift, gifting: "The greatest gift of my generation is that a human being can alter his life by gifting his attitude." This option doesn't fit logically, as 'gifting' an attitude does not make coherent sense.
- discovery, altering: "The greatest discovery of my generation is that a human being can alter his life by altering his attitude." This choice is logical and fluent; 'discovery' indicates a key realization, and 'altering' accurately describes changing one's mindset.
- misgiving, elevating: "The greatest misgiving of my generation is that a human being can alter his life by elevating his attitude." While 'elevating' could fit, 'misgiving' suggests doubt rather than progress, making this illogical.
- thing, flaunting: "The greatest thing of my generation is that a human being can alter his life by flaunting his attitude." 'Thing' is vague, and 'flaunting' implies showing off rather than changing.
Therefore, the correct words to complete the sentence are: discovery, altering.