The correct option is
A We
Option A: 'We' is a subject pronoun that performs a function. It relates to the given sentence; 'we' performs the function 'forget'.
Hence option A is correct.
Option B: 'As' indicates comparison between two things. The meaning doesn't apply here.
Option C: 'To' indicates a movement or direction towards a point or person.
Option D: 'In' means 'inclusions within a space'.
These words do not complete the sentence in meaning.
Hence options B, C and D are incorrect.