Final answer:
The correct option is to use a semicolon to join the two clauses.
Step-by-step explanation:
The correct option is:
A filmy white ghost slithered out of the gravestone; it slowly floated towards me.
This option correctly uses a semicolon to join two independent clauses, showing that they are related but separate thoughts. The other options incorrectly use either a comma splice or no punctuation at all between the two clauses.
Learn more about Correct use of punctuation in joining independent clauses.