Answer : 4. "to seeing"
Explanation :

• In this case “to” is a preposition, and you always need to use the ‘ing’ form (gerund) after a preposition.

• ‘looking forward’ is a phrasal verb. A phrasal verb contains a verb and a preposition and in this case, two prepositions (forward and to).

• To look forward to is a transitive verb which means that it needs an object ( a noun or a verb).

• As ‘seeing’ is a gerund (noun form of verb), the answer should be ‘to seeing’.

• This option uses the gerund form “seeing” after the preposition “to.” It is the most appropriate choice as it maintains parallel structure and conveys the idea of anticipation or expectation. “Looking forward to seeing” is a common phrase used to express excitement about a future event.