The principle says the communication of a proposal is complete only when it reaches the knowledge of the person it is made to, not merely when it is sent or physically delivered. A sent a letter to B proposing to buy B's house. Let's check each option against this principle.
Since the principle ties completion of communication to the proposal reaching the recipient's actual knowledge, it is only complete once B reads the letter himself.
Therefore, the correct answer is the communication of proposal is complete when B reads the letter.
The principle can be read as a conditional rule: communication of a proposal is complete only at the moment its contents actually reach the knowledge of the person to whom it is addressed, not at any earlier point in the letter's journey. A sent a letter to B proposing to buy B's house, and the letter passed through B's wife before reaching B. We can test each candidate moment in that journey against this one condition: has the proposal's content actually reached B's own knowledge yet?
Testing each stage of the letter's journey against the single condition of reaching B's actual knowledge shows that only the moment B reads the letter satisfies it.
Therefore, the correct answer is the communication of proposal is complete when B reads the letter.