Step 1: UN Formation Year
The United Nations was officially established on October 24, 1945, following the end of World War II.
Step 2: Charter Signatories
The founding members are those countries that signed the United Nations Charter at the San Francisco Conference in 1945.
Step 3: Total Number of Founding Countries
Exactly 51 countries were original members who ratified the charter, thus becoming the founding members.
Step 4: Conclusion
Therefore, the correct number of founding countries is 51.