Option 1: actually, but -> "actually" emphasizes the statement while "but" introduces a contrast between being invisible and suddenly appearing.
Option 2: alternatively or -> suggests alternative choices rather than contrasting behaviors, which doesn't fit the context.
Option 3: consequently and -> implies the appearance is a consequence of being invisible, which is illogical.
Option 4: neverthless, so -> combines contrast (nevertheless) with causation (so), creating a contradiction.
Hence, Option 1: actually, but -> The sentence needs a conjunction that shows contrast between two behaviors: being invisible for years versus appearing suddenly. "Actually" emphasizes the reality of the first part, while "but" properly introduces the contrasting behavior that follows -> correct