Question

but it raises the specter of isolation.
in this sentence, specter means:
category name
a pattern of symptoms indicative of some disease
the visual percept of a region
a mental representation of some haunting experience

Explanation:

The word "specter" has the meaning of something that haunts or disturbs one's mind. In the context of the sentence "But it raises the specter of isolation", it implies a mental representation of a disturbing situation (isolation).

Answer:

a mental representation of some haunting experience