Final answer:
As large Roman landowners gained economic power, small farmers were forced off their land, the unemployed population increased, and the Republic declined, showing that all the listed options occurred as a result of this economic shift.
Step-by-step explanation:
As large Roman landowners gained economic power, several consequential events followed. Small farmers were forced to sell their land, often because they couldn't compete with vast, slave-tilled plantations or because of outright extortion by wealthier landowners. This displacement contributed to a significant increase in the population of unemployed individuals, swelling the ranks of the proletariat in urban centers like Rome. Additionally, the traditional Roman political system began to falter, leading to a decline in the Republic itself, as politicians adjusted their strategies to garner the support of the landless masses, resorting to practices like offering free food, entertainment, and the promise of employment through public works.
All these factors combined indicate that the answer to the question, 'Which of the following occurred as large Roman landowners gained economic power?', is D. all of the above, as each of these developments played a role in the transformation of Roman society and the ultimate decline of the Roman Republic.