Concept:The blank needs a relative clause describing "people".
Explanation:The phrase "one of the kindest people" is followed by a relative clause.
When the relative pronoun is the object of "know", it can be omitted.
So "people I know" is the correct and natural completion.
Option A is wrong because "I" takes "know", not "knows".
Option C is wrong because "who" is not used as the object here; "whom" or omission is needed.
Option D is wrong because "which" is not used for people.
Option E changes the tense unnecessarily.
Answer:Option B: I know