The correct answer is option 'C' - every 30 years.

Under the Ryotwari system, which was implemented during the British colonial rule in India, the lands were resurveyed every 30 years. This system was introduced by Thomas Munro, the then Governor of Madras, in the early 19th century. The purpose of the resurvey was to establish a more efficient system of revenue collection and land ownership.

Explanation:
Under the Ryotwari system, the land was directly leased to individual cultivators or ryots. This was in contrast to the earlier zamindari system, where the land was leased to intermediaries known as zamindars, who were responsible for collecting revenue from the cultivators. The Ryotwari system aimed to eliminate the zamindars and establish direct contact between the British government and the cultivators.

Resurveying the lands:
To implement the Ryotwari system effectively, it was necessary to conduct regular surveys of the land to determine its boundaries, assess its quality, and fix its revenue. The resurvey was carried out every 30 years to ensure that the revenue assessments were updated and accurate.

Importance of resurvey:
The resurvey was crucial for several reasons:

1. Revenue assessment: The resurvey helped in determining the appropriate amount of revenue to be collected from each piece of land. It took into account factors such as soil fertility, water availability, and other agricultural conditions. This ensured that the revenue demand was fair and based on the actual productivity of the land.

2. Land ownership: The resurvey also helped in establishing clear boundaries and ownership rights. It prevented disputes and conflicts over land ownership by providing accurate records of land boundaries.

3. Modernization: The resurvey was part of the larger effort to modernize the revenue administration in India. It introduced standardized measurement techniques and documentation practices, which improved the efficiency and transparency of the system.

4. Updating land records: The resurvey also involved updating land records, which included information about the landholders, their rights, and the revenue payable. This ensured that the records were up to date and reflected the current state of land cultivation.

In conclusion, under the Ryotwari system, the lands were resurveyed every 30 years. This resurvey was an essential component of the system, as it helped in determining revenue assessments, establishing land ownership, modernizing the revenue administration, and updating land records.