Answer:
A) I forgot
Explanation:
The best way to find this answer would be to remove the phrase "it slipped my mind" in sentence four and replace it with each of the options.
C and D don't really apply to the context of the paragraph, so we're left with A and B.
A is the best answer, due to the fact B seems like the author is dismissing it, which wouldn't make sense due to the fact they apologize to their sister in the beginning of the paragraph.
The author also lists reasons why they forgot in the beginning of sentence four, which adds to the validity of answer A.
