Correct option is A
The most appropriate option that can substitute the highlighted segment in the given sentence is (a).
(a) Has gone - a past participle of the verb "go" used in the present perfect tense to describe an action that happened at an unspecified time before now.
(b) Are gone - a present tense of the verb "go" used in the plural form to describe an action happening at the present time.
(c) Gone - a past participle of the verb "go" used in the present perfect tense to describe an action that happened at an unspecified time before now.
(d) Were gone - a past tense of the verb "go" used in the plural form to describe an action that happened at a specific time in the past.