The correct option is A am supposed to see
Let’s examine the sentence: “I _______________ him at 10:00 a.m. tomorrow.” The use of “I” demands the use of helping verb “am.” This rules out Option C as it does not have ‘am.’ All the options use ‘suppose,’ however when used with ‘am,’ it would be used in its third form ‘supposed.’ This rules out Option D as it does not have ‘supposed.’ Option B cannot be used as ‘to seeing’ is incorrect English. Hence, Option A is the correct answer.