Final answer:
The Rashidun caliphs reigned for about 29 years (632-661), with the closest choice from the provided options being 'about 25 years', although it is slightly inexact.
Step-by-step explanation:
The early caliphs, known as the Rashidun, reigned before the first fitnah for about 29 years, from 632 to 661. These caliphs were pivotal in the early expansion of the Islamic empire and included Abu Bakr, Umar, Uthman, and Ali. The period they ruled over is marked by significant territorial expansion and the spread of Islam.
Nevertheless, it ended with the first fitnah, or civil war, which started after the murder of the third caliph, Uthman, and continued through the reign of the fourth caliph, Ali, culminating in his assassination in 661.
Based on the information provided, the closest answer to how long did the early caliphs reign before the first fitnah is about 29 years, which is not explicitly listed among the choices given (a. about 15 years; b. about 45 years; c. about 35 years; d. about 25 years). If you must choose from the provided options, the closest correct choice would be d. about 25 years, acknowledging that it is approximately four years short of the actual period.