Sentence Correction

Q: Certain gerentologists have reported that the more older people continue to challenge their brains with reading, writing, and other thought provoking execises, their cognitive functions are less likely to diminish.

b)the less likely are their cognitive functions to be diminshed.
c)the less are their likely to have diminished cognitive function
d)the less likely their cognitive functions will diminsh
e)they are less likely to have dminished cognitive function

I feel the aswer should be B or C

Source: Kaplan GMAT 800

Answer:E

b)the less likely are their cognitive functions to be diminshed. : are and to be ---two working verbs
c)the less are their likely to have diminished cognitive function : Awkward construction