答案应选 killing。

句中的 killing two birds with a stone 为动名词短语，用作宾补。句意：

People call that killing two birds with a stone. 人们称之为一石二鸟。

类似的例句如：

I call this robbing Peter to pay Paul. 我管这叫做拆东墙补西墙。

Do you call that being at attention? Stand up straight! 你这也叫立正？给我站直了！

Only six months in prison for murder—I call that getting off lightly. 犯谋杀罪只蹲六个月监狱——我说那等于让凶手逍遥法外!

You always wake early in the morning, so you can’t claim it as a moral advantage; I call that making a virtue of necessity. 你早上总是醒得早，所以你不能说这是品德上的优点，我把它叫做顺应现实。