关于英语的问题，求解答，谢谢！

if当是否讲时， 只引导宾语从句，并且是动词的宾语从句，介词的宾语从句都用whether，这里depend on 根据的意思，on是介词，后面要选的词需要做on的宾语，因此不能选if。

C is correct. I pick this one is because 'if' and 'whether' can be interchangable in many situations. In this case, it is more formal if I use 'whether'. However, when you speak this sentence, it makes like a tongue twister, therefore, when you listen to conversations, equal number of people use 'if' to avoid two similar sounding words.
The reason whether is the grammatically corrected one is because there is a clause following. Picking C is my habit of speech.

选 B.whether
意思是这事要随着天气是否晴朗而定。
介词后面只能用 whether不可用 if

你好！

在介词后用whether，而不用if；

例如：It depends on whether he can solve the problem. 那取决于他能否解决这个问题。

取决于是否好天气。whether the weather is fine (or not).

这里省略了 or not