Blackbeard first went to sea A when he was quite young. B to work for Pirate Hornigold. C to fight the French. D to sail to Spain.

The text states: "Teach began his career on the high seas at an early age when he sailed in the English Navy in the War of the Spanish Succession (1702-1713). After the war he joined the crew of the Pirate Hornigold." This implies he went to sea at a young age and then worked for Hornigold.

Therefore, the most accurate answer is:

A when he was quite young.