The question asks to identify the correct statement concerning agreements and the concept of impossibility.

Identifying Void Agreements: Impossible Acts

The core principle here relates to the validity of agreements based on the possibility of performing the agreed-upon act. According to contract law principles (often derived from statutes like the Indian Contract Act, 1872, Section 56), certain types of agreements are inherently invalid from the start.

Analysis of Statements

• Statement 1: "An agreement not to do an impossible act in itself is void." This is incorrect. An agreement *not* to do something impossible is often valid, as the impossibility doesn't hinder the agreement itself.
• Statement 2: "An agreement to do a possible act is void." This is fundamentally incorrect. Agreements to do possible acts form the basis of valid contracts.
• Statement 3: "An agreement to do an act impossible in itself is void." This statement correctly reflects the legal principle. If an act is impossible from the very beginning (impossible in itself), any agreement based on performing that act cannot be enforced and is considered void ab initio (from the beginning). Examples include agreeing to discover treasure by magic or agreeing to pay money to a person who is already dead.
• Statement 4: "A contract to do an act, which after the contract is made, becomes impossible... becomes valid, when the act becomes impossible or unlawful." This statement is incorrect. When an act that was initially possible becomes impossible *after* the contract is made (supervening impossibility), the contract typically becomes void, not valid.

Conclusion on Impossibility

Based on the analysis, the only statement that accurately represents a fundamental rule of contract law regarding impossibility is that agreements whose subject matter involves an act impossible in itself are void.