The correct linking words to indicate sequence are "as" and "first."
Both are connectors that join different parts of a text and provide the written work with a chronological order.
The other alternatives are wrong because of the following reasons:
- "When" is a conjunction that means "just the moment that." "Tubby" is the name of the baby raccoon, so it does not indicate sequence.
- "Finally" is a connector which shows progression, but its pair "ready" is an adjective.
- "Always" is an adverb of frequency, and "because" is a conjunction that suggests reason.