A和D

问题如下：

Negative vega probably occurs when:

选项：

A.

a forward start option just start the existence.

B.

the holder of a chooser option determines to choose the option is a call.

C.

the holder of a shout option "shout" to the writer.

D.

a down-and-out put when the price of underlying asset is close to the barrier level.

解释：

D is correct.

考点：Exotic Option

解析：Vega衡量的是期权价格随股票波动率而变化的敏感程度。大多数期权的vega是正的，但是有一个特例：敲出期权knockout barrier option。当波动率增加，触碰到barrier level的可能性增加，那么knockout barrier option作废的可能性增加，所以这种期权越不值钱，因此价格下跌，造成negative vega。