The given sentence states an action that took place in the past "a week ago".
Option C: 'Saw' is a past tense verb and states an action that happened in the past: "a week ago". Hence option C is correct.
Option A:
'Would see' is a conditional. It states an action that may or may not happen. But, the action has already happened: "Yes, I have."
Option B: 'Had seen' is in the past perfect tense. It states an action that happened in the past, before another action began.
But here, only one action has taken place 'to see the film'.
Option
D: 'Have seen' is in the present perfect tense. It states an action that has just happened in the present. But the action here has already been completed in the past "a week ago".
Hence options A, B and D are wrong.