Final answer:
A is entitled to redeem only his undivided interest in the lot, equivalent to his share before the sale, due to his reservation of the right of conventional redemption.
Step-by-step explanation:
If A exercises his right to conventional redemption, he is entitled to redeem his undivided interest in the lot, equivalent to his original share prior to the sale to X. The principles of justice in acquisition and in transfer suggest that one is entitled to a holding if it is acquired justly from someone who is entitled to that holding. Since only A reserved the right of redemption, the redemption would be limited to his share, and he would not have a claim to the shares formerly owned by B and C, as they did not reserve any such right.