The correct options are
B They can be replaced by any other labour, ready for lesser pay.
C They don't have a stable source of income.
Agricultural labourers are very poor, and unable to make two meals a day without farming work. People sometimes manage other income source but even that doesn't give them enough money. If they ask the landlord for a hike (increase in pay), they can be replaced and they are never forced by the landlord to work but it is their own choice to work under the landlords.