Concept:The expression "count on" means to rely on or depend on someone. It is the correct phrasal verb for seeking dependable support.Explanation:The sentence asks if the speaker can rely on the listener's help.The word "count" is usually followed by "on" when the meaning is "to depend on".The other prepositions do not form a standard phrase with this meaning.Therefore, "on" fits the blank naturally and grammatically.Answer:Can I count on your help?