Introduction:
By the 1850s, Britain underwent significant infrastructure development, particularly in transport. This period saw the expansion and integration of railways across the country, connecting various regions and facilitating the movement of goods and people. As a result, railways became the most prominent mode of transportation in Britain during this time.

Explanation:
The correct answer to the question is option 'B', railways. Let's delve into the reasons behind this:

1. Railway Boom:
The mid-19th century witnessed a rapid expansion of railways in Britain, known as the "Railway Boom." The government encouraged the construction of railways through various acts and policies, leading to a surge in railway development. This extensive network of railways connected most parts of Britain, making it the primary mode of transport.

2. Improved Connectivity:
Railways provided a reliable and efficient means of transportation, connecting various towns, cities, and industrial areas. This connectivity brought people closer and facilitated the movement of goods, raw materials, and finished products across the country. As a result, railways played a crucial role in promoting trade, commerce, and industrial growth.

3. Economical and Faster Travel:
Railways offered a much faster and more comfortable mode of travel compared to other means available at the time, such as canals or roads. The introduction of steam locomotives and the development of railway infrastructure allowed trains to travel at higher speeds, reducing travel time and making journeys more convenient. Additionally, railways provided a cost-effective mode of transportation, making it accessible to a larger population.

4. Industrial Revolution:
The Industrial Revolution, which occurred during the 18th and 19th centuries, brought about significant changes in Britain's economy and society. Railways played a crucial role in supporting and driving this industrial growth by providing a reliable transportation network for raw materials, goods, and workers. The integration of railways into the industrial landscape made it an indispensable part of Britain's infrastructure.

Conclusion:
By the 1850s, the majority of Britain was connected by railways, making it the most widespread and prominent mode of transportation. The extensive railway network improved connectivity, facilitated economic growth, and played a vital role in supporting the Industrial Revolution. This transformation in transport infrastructure revolutionized travel and trade, shaping Britain's development during the 19th century.