An entertaining quiz with much amibiguity here, rather like the classic (punctuation) book Eats shoots and leaves, ( a man walks into a bar, eats, shoots and leaves, and then a panda ambles into the same bar and eats shoots and leaves).
You can make a few simple inferences from the sentences, but that is all. Thus
a) The single article 'a' implies the man and woman were a single object (in the subject-verb-object sense), thus the subject spoke to both of them on the same occasion, however it is not clear whether they spoke to both persons individually or as a pair. Their marital status is also unclear, however the subject would probably have described them as a couple if they knew them to be close.
b) As with (a), the lean man and woman were met on the same occasion, but it is ambiguous who was spoken to and is not clear whether the woman was lean
c) The use of two articles 'a' implies the man and woman were two separate subjects. However it is not clear whether they were spoken separately at different times, or at the same time (and if the latter then whether both were individually spoken to or not).
d) In this case the two separate articles differentiate the man as lean and the woman as not lean, however it still does not clarify who was spoke to and when, as with (c)