8 .
Only ONE of these versions will NOT make clear sense as a completion of the sentence: which one is it?
'As it turns out, we didn't need the report until next week,' commented his line-manager the following morning, ... ...
... 'but I am none the less grateful to you for putting in such an effort to get it ready ahead of time.'
... 'but I am grateful to you regardless, for putting in such an effort to get it ready ahead of time.'
... 'but I am nonetheless grateful to you for putting in such an effort to get it ready ahead of time.'
... 'yet I am grateful to you all the same for putting in such an effort to get it ready ahead of time.'
None of the other Answers makes plausible sense in the circumstances; check them again!