The correct answer is option 'A', which refers to the peasants who traveled long distances on pilgrimage, passing through towns.

Reasoning:
Pilgrimage was an essential part of medieval Christian life, and it involved traveling to holy places to seek spiritual benefits. The journey was usually long and arduous, and the pilgrims had to pass through various towns and cities on their way. The following points explain why peasants were the primary pilgrims who traveled long distances on pilgrimage, passing through towns.

1. Availability of time: Peasants were the most significant social group in medieval Europe, and their lives were governed by the agricultural cycle. However, they had a few periods of leisure, such as the winter season, when there was less agricultural work. This time was ideal for undertaking pilgrimages, which could take several months.

2. Religious fervor: The medieval society was deeply religious, and the peasants were no exception. They believed that undertaking a pilgrimage was a way of atoning for their sins and earning spiritual merit. Therefore, they were willing to endure the hardships of the journey.

3. Economic affordability: Pilgrimages were not cheap, and the cost of travel, food, and lodging could be significant. However, the peasants could afford to undertake pilgrimages since they did not have to pay taxes or rents on their land. Moreover, they could rely on the hospitality of the monasteries and churches along the way.

4. Social status: Peasants were at the bottom of the medieval social hierarchy, and they had limited opportunities for social mobility. Undertaking a pilgrimage was a way of asserting their religious identity and gaining recognition from their peers.

Conclusion:
In conclusion, peasants were the primary pilgrims who traveled long distances on pilgrimage, passing through towns. Their availability of time, religious fervor, economic affordability, and social status were the main reasons why they undertook pilgrimages.