**Introduction:**
In this scenario, a boy of 21 years for whom a guardian has been appointed enters into a contract with B regarding his property. The question is whether the contract is valid, voidable, void, or invalid.

**Explanation:**
To determine the validity of the contract, we need to consider the legal capacity of the boy and the authority of the guardian. Let's analyze the options:

**A) Valid:**
If the boy has attained the age of majority (which is generally 18 years) and the guardian has the legal authority to act on his behalf, the contract would be considered valid. This means that the contract is enforceable by law and both parties are bound by its terms and conditions. In this case, the boy being 21 years old indicates that he has reached the age of majority.

**B) Voidable:**
A contract is voidable when one party has the option to either enforce or reject it due to certain legal flaws or deficiencies. However, in this scenario, there is no indication of any legal flaws or deficiencies that would render the contract voidable.

**C) Void:**
A contract is considered void when it lacks legal enforceability from the beginning. In this scenario, since the boy is of legal age and a guardian has been appointed, there is no inherent reason for the contract to be void.

**D) Invalid:**
A contract is considered invalid when it lacks the essential elements required to create a legally binding agreement. However, in this scenario, there is no indication that the contract lacks any essential elements.

**Conclusion:**
Based on the given information, the correct answer is option 'A' - the contract is valid. This means that the contract is legally binding on both parties, and the boy and B are obligated to fulfill their respective obligations as per the terms of the contract.