"It is difficult"
Just as in English, adverbs are sometimes created from adjectives. In Swahili, there are some adverbs that take on what appears to be agreement with the KI- VI- noun class.
Vibaya - badly
Kidogo - a little, slightly
Vigumu - difficult
Vizuri - well, nicely
So if the adverb has an adjective stem, a good rule of thumb is to assume ki/vi.
That said, "difficult" is not an adverb, but is actually an adjective, so I don't really know how the rule applies here (hopefully someone with a bit more knowledge of grammar rules can weigh in).
If in doubt, I tend to assume ki/vi when the subject is undefined, as the word for "thing(s)" is kitu/vitu. Might not always be marked right on Duo though.