As per the provisions of the Indian Evidence Act, 1872, if a contract is contained in several letters, what must be proved ?

According to Section 91 of the Indian Evidence Act, 1872, when the terms of a contract, grant, or any other disposition of property have been reduced to the form of a document, no evidence shall be given in proof of those terms except the document itself.

Explanation 1 of Section 91 clarifies that this rule applies whether the contract is contained in a single document or multiple documents. Crucially, Illustration (a) to Section 91 explicitly states: "If a contract be contained in several letters, all the letters in which it is contained must be proved."

This requirement ensures that the court has the complete correspondence to accurately determine the final agreement and the intentions of the parties, rather than relying on isolated or incomplete parts of the communication. Therefore, option C is the correct legal position.