Answer: to seeing
Step-by-step explanation:
It is important to note that infinitive does not usually act as a verb despite the fact that it is being formed from a verb. We should note that it acts as a noun, an adjective, or an adverb.
Infinitive typically consists of two words which are: to + verb. Therefore, based on the question given, the answer will be:
= to + seeing
= to seeing
We look forward to seeing you at the weekend.