The law of proximity states that any subject closest to the verb element controls the verb form.

The sentence has a singular subject 'it' and a plural subject 'you', hence a plural verb is needed because 'you' is closer.

Similarly, 'supposed' is most appropriate because of the relative clause 'that'.
This implies that supposed functions as an 'adjective' in this context which means 'intended'

Thus, Option D is the correct answer.