Correct Option (A)
Assumption I is valid. The passage explicitly states that achieving even half the potential reduction through demand-side measures, such as behavioural changes, would necessitate companies and governments providing incentives via supply-side investments. These investments are intended to make low-carbon options more affordable and widely available. This directly implies that supply-side investments are considered instrumental in enabling and encouraging low-carbon behaviour among individuals.
Incorrect Options:
Assumption II is not valid. The passage suggests that significant demand-side behavioural change requires the involvement of government and companies to provide incentives. However, it does not assert that individuals are entirely incapable of adopting any low-carbon behaviour without such external involvement. The text questions the sufficiency of purely voluntary behavioural change for achieving substantial emission reductions, rather than denying the capacity for any adaptation whatsoever. The term "not capable" makes the assumption too absolute and is not supported by the passage.
Therefore, options B, C, and D are incorrect because Assumption II is not a valid inference from the provided passage.