The given sentence is a type 3 conditional, as the clause "she would have helped..." presents a probable past result to an unreal past condition 'if she had known about...".
So, the if-clause will be in the past perfect tense. 'Had known' expresses the past perfect tense and hence option C is correct.
Options A, B and D: 'Knew' is a simple past tense verb, 'had been knowing' expresses the past perfect continuous and 'have known' presents the present perfect. All these verb forms do not express the unreal past condition in a type 3 conditional.