你的问题应该是这样的吧?
Since she is angry,we ______.
a.had better leaving her alone b.should leave her alone
c.might as well leave her alone d.had rather leave her alone
答案：可以用排除法,我的分析是这样的
A：因为had better的用法是 had better+do 后不应该用现在分词,
应该是had better leave her alone就对了
B：意义不对,这里使用should的话表示了应该去做没有做过的事情
比如：You should complete your test in time.你们应该及时完成你们的测试.（但实际没有完成）.可以看到,用进去意义就不对了.
C：might as well 还可用于指过去的情况或用于比较两个令人不愉快的情况,强调前后主语要一直另外是不愉快的情况.所以不对.
D：had rather表示宁可,宁愿
比如：I had rather join you in research work than go on holiday to the seaside.
我宁愿和你们一起参加科研工作,而不愿去海滨度假.
所以原句的解释应该是
因为她生气了,所以我们宁可让她一个人待着