Pahom is unhappy.
A linking verb links a subject to its description. A linking verb is not an action (something you can do). The subject of all the sentences is Pahom. The last three options all have action verbs (moves, buys, dies). These are all things you can do. In the first option, is is the verb and it's not something you can do. Some people try to argue that unhappy is something you can do and therefore might be a verb, but it is not. Unhappy is something you can BE. This is the key difference. Unhappy is an adjective that describes Pahom. Is links the adjective and subject together.
