You'd better__ electric wires are safe and that the childran are safe.

C make sure 表动作，也是固定搭配。此句中make sure 连了两个从句。
意思为：你最好确定电线是安全的以及孩子们是安全的。

C. make sure

You'd better__ electric wires are safe and that the childran are safe.
A. make sure of B. be sure of C. make sure D. find
make sure (that)+从句“确保，保证”

C嘛！！！