The correct option is A saw
The given sentence states an action that happened in the past 'last week'. Hence the correct form of the verb must be in simple past tense.
Option A: 'Saw' is a simple past tense verb and indicates an action that complete in the past. It's relevant to the sentence and hence option A is correct.
Option B: 'Have seen' is in the present perfect tense. It indicates that an action has just been completed.
Option C: 'Were seeing' is in the past continuous tense. It indicates that an action was continuing during a certain point of time in the past. The action of seeing a movie is not described as a continuous one here. It simply states an action that completed.
Option D: 'See' is in the present tense and indicates a daily habit. The action here isn't a daily habit.
Hence options B, C and D are incorrect.