Correct option is A
The correct substitution for the highlighted segment is (a) "have shown unwillingness."
"Have shown unwillingness" correctly reflects the guests' general disposition towards attending the event, expressed in a grammatically correct form.
Grammatical rule used: "Unwillingness" is the noun form that fits the structure of the sentence, indicating the state of being unwilling.
Example: Despite the incentives, there was significant unwillingness among employees to relocate.