Concept:The expression "look forward to" is always followed by a gerund, which is the "-ing" form of a verb.
Explanation:In the phrase "look forward to", the word "to" is a preposition.
After a preposition, a verb must be changed into its gerund form.
The gerund form of "see" is "seeing".
This is similar to common correct usage such as "I look forward to hearing from you."
Therefore, the completed sentence is: "Remy looks forward to seeing Jane at the party."
The other options, "see", "have been seeing", and "be seeing", do not follow the required gerund pattern.
Answer:Option A: "seeing"