The correct option is A her leaving
Sentence B needs to state the fact of Helen leaving, where leaving is to be taken as gerund because it points towards her situation of leaving (noun) and not action of leaving (verb). The gerund always has the same function as a noun (although it looks like a verb). The gerund ends in ‘ing.’ This rules out Options C and D. Option B also cannot be used as ‘she leaving’ is wrong English. So, Option A is the correct answer.