Final answer:
Romeo had to leave Juliet at dawn to avoid the death penalty for being in Verona after banishment, and Juliet feared the repercussions if their union was discovered due to their families' feud.
Step-by-step explanation:
Romeo had to leave quickly when the sun rose because being discovered in Verona after daybreak could lead to his death, as he was banished and presence in the city considered a capital offense. Juliet's fear was that their families' feud would lead to disastrous consequences if they were found together. Hence, the correct answer to why Romeo had to leave so quickly when the sun rose and what Juliet feared is: (a) Romeo had to go before they were discovered, and Juliet feared their families' feud.