B

问题如下：

Following are some regulatory restrictions, which helps improve market efficiency?

选项：

A.

restrictions on short sell.

B.

restrictions on foreign traders.

C.

restrictions on insiders trading with nonpublic information.

解释：

C is correct.
By imposing regulatory restrictions on insiders to use unfair nonpublic information, the market would attract more participation, and the expansion of market players would help increase market efficiency.

考点：Efficient Capital Market And Its Forms

限制内幕交易使得市场参与者能更加公平的进行交易，市场有效性增加。