Final answer:
The circumnavigator who reached the Pacific in August 1578 with only his flagship left was Sir Francis Drake. He completed the second circumnavigation of the world from 1577 to 1580.
Step-by-step explanation:
The circumnavigator who reached the Pacific in August 1578 with only his flagship remaining of the 5 that left England was Sir Francis Drake. He is known for completing the second circumnavigation of the globe in a single expedition from 1577 to 1580. His journey was marked by numerous challenges, including hostile encounters with the Spanish and the loss of his accompanying ships save for his flagship, the Golden Hind.