The correct answer is a.: specific.
This structure (this +adj) requires an adjective and the only adjective is in a., the other options are verbs or nouns and are not correct.
By saying that, he indicates that they know each other very well and that without talking, they have similar ideas and outlooks on the world in response to certain situations.
The answer for this i think is subordnate clause
1) When did she come home yesterday
2)Do you go to school on Saturday
3)Perhaps tomorrow it won't rain
4)Did they read that book
5)Has he got a new car
6)Will you buy her a present
Because the x was this and plus that and plus all that other stuff
