In its early stages, dementia is often _______________ from normal ageing, in which the older brain may naturally begin to show signs of cognitive impairment. Neither dementia nor cognitive ageing is currently _____________, though they may be slowed through diet, exercise, and keeping the mind agile with social and intellectual activity.

For the first blank, we need a word that conveys that dementia, in its early stages, is not easily told apart from normal ageing. For the second blank, we need a word that indicates neither condition can be fully undone, though they can be slowed through healthy habits.

Indistinguishable is the best choice for the first blank, as it means dementia and normal ageing appear very similar early on. Reversible is the best choice for the second blank, as the sentence says these conditions cannot be reversed, only slowed.

Hence, option B is the best choice.