Answer:
Mostly money and promise of the American dream.
Many poor Europeans and other immigrants heard about the Americans and their lavish lives and wanted to try their luck and escape their poverty. However, they didn't have enough money to emigrate to America which is why they would become indentured servants.
This meant that they would sign a contract with their employer who would then pay for their travel expenses to America, and in turn they would have to work for them for a number of years in return for food and shelter. When the contract expired, ideally the indentured servant became free (although this didn't always happen). Their status was slightly better than that of a slave.
(Since you didn't give us any options, I can't tell you the exact answer).
