SO, ATTEMPTING TO MOVE ON ...
Assuming 'spelling' is uncountable when the adjective is 'difficult'.
Doesn't that mean both (a) and (c) need to use 'spelling', not 'spellings'?
Which (tentatively) gives us:
(a) The spelling of homophones is difficult.
(c) The spelling of 'their' and 'there' is difficult.
BUT ... aren't uncountable nouns sometimes treated as either singular or plural, depending on whether the action was collective or individual.
Writing that reminded me of the answer. Yes, "mass nouns" are always singular, but collective nouns may be treated as either. :-)
So, are these examples mass uncountable nouns or collective uncountable nouns?
I'm not sure, but (c) definitely feels like two individual actions, suggesting the noun be treated as plural.
OTOH, (a) definitely feels like a mass action, suggesting it should be singular.
That would make the four sentences required:
(a) The spelling of homophones is difficult.
(b) The spellings of homophones are different.
(c) The spelling of 'their' and 'there' are difficult.
(d) The spellings of 'their' and 'there' are different.
BUT ... reading those the one that sounds off to me is (c), which sounds like it needs 'is'. Going back to AJ's first post, when he suggested the order of the verbs should be is, are, is, are, his reason was the two words would be treated as a pair that is difficult to spell. That sounds fair enough to me.
That would make the four sentences required:
(a) The spelling of homophones is difficult.
(b) The spellings of homophones are different.
(c) The spelling of 'their' and 'there' is difficult.
(d) The spellings of 'their' and 'there' are different.
Does anyone have any problems with those?
AJ, thanks for the help and I'll give you a B grade. You got the verbs right, but missed (b) and (d) were countable nouns and needed 'spellings'.
The rest of you were no help at all, and everyone who made a guess, plus me, failed miserably.