How does Kaspa determine which block reward transaction is the valid one?

Kaspa validates exactly one reward transaction per mined block — the one issued by the child block that ends up on the "selected chain." The selected chain is the single authoritative path through the blockDAG, sometimes called its "barebone" — the spine that the whole network agrees on after consensus is reached. Because Kaspa is a blockDAG and not a single-file chain, multiple blocks can follow your block simultaneously, each initially issuing you a reward. Once consensus settles the ordering, only the reward from the selected-chain block survives; all others are discarded. Understanding this matters because it explains why explorers may show more reward entries than you actually received — the extras are real transactions that were overridden when the network agreed on the final chain.