Answer:
When the matching funds are raised.
Explanation:
Since in the question it is mentioned that a benefactor vow to denote $30,000 for his church in order to buy the piano but the church should capable to increase the matching fund for $30,000 from other contributors
So at the time of recording the revenue by the church when there is a raise in the matching fund otherwise, it would not be recorded
Therefore the last option is correct