怎么说呢,换个词解释就清楚了,例如：
I tell him to do his homework.我叫他做作业（暂不考虑时态）.
有两种否定：I tell him not to do his homework.和 I don't tell him to do his homework.
区别除了not位置不同,就是意思了.前者“我让他别做作业”,后者“我没让他做作业”.听中文应该能听出有区别吧.
所以说这个题目出的很不好,两种否定都可,建议你选第一种 （don't decide to do my homework）