The correct form is assistance.
"Please do not hesitate to contact me if there are any further questions or if I can be of assistance in any way."
The blank follows the preposition "of," which requires a noun. The noun form of "assist" is assistance. "Assistive" (adjective) or "assisting" (verb) would be grammatically incorrect here. This is a standard formal letter closing phrase, and recognising noun forms from given base words is a common word-form exercise in CBSE exams.