Which of the following is necessary for the competency of parties in a contract?

For parties to be considered competent in a contract, they must possess the legal capacity to enter into agreements. This means they should be of legal age, generally understood as 18 years or older in most jurisdictions, and must have the mental ability to understand the terms and consequences of the contract. Legal capacity ensures that individuals have the rights and abilities defined by law to engage in contractual relationships, making it a fundamental requirement for enforceability.

The other options do not universally pertain to competency in contractual agreements. A minimum age of 30 is not a standard requirement, as the legal age for contracts is typically 18. Similarly, having a common educational background or previous business experience may enhance one's ability to negotiate or understand contracts but is not a legal requirement for competency. Therefore, the necessity of legal capacity is the correct foundation for determining the competency of parties in a contract.