• was seeking vs. have sought

Hi!
I read the following in a TOEIC book:

“Question 143 refer to the following e-mail.

Dear Mr. Lau,

Reliquary Antiques has a strict set of requirements for items that we purchase for resale. Nonetheless, we accept many items in their current state, unless the passage of time has caused significant physical damage. When such damage is present, our experts will evaluate that item. Their knowledge is unparalleled, as is the expertise of our restoration team. If the damage does not compromise the piece’s structural integrity, we will often accept it.

I have recently gained a customer who collects furniture like yours, and I [143]_______ pieces for him. Therefore, I am happy to inform you that despite the visible damage to the chair, we will be glad to purchase it from you. Please contact me so that we may set up a meeting to discuss the sale. Thank you.

Sincerely,

Wallace Sterling
Head of Acquisitions
Reliquary Antiques

[143]
(A) seeking
(B) sought
(C) was seeking
(D) have sought”

((PAGODA TOEIC 1000 questions, Reading Comprehension, Test 5))

I think C is the best answer, but I want to know your opinions.
Which one do you think is the best answer?
Thanks in advance.