Japanese sentence question

Hi,

Someone please explain to me this sentence.

あれ。私のジュースがない。さっきここに？？？のに。。。

A. 置いといた
B. 置いてみた
C. 置きかけて
D. 置いちゃった

The correct answer given was A. The explanation was that 置く+ ておく. But then, how did it become とおいた？Why not just 置いておいた？
And also, please explain the usage of ておかない and give some example if you can. :D

Thanks.

I think 置いといた is shortened from 置いておいた.

It’s a natural dropping off (elision) of the vowel e like in French.
-te oku > t’oku > toku
It’s a frequently occuring sound change in a casual Japanese speech.

In the first question (the sentence of the exercise), two おく are used. The first, 置く(oku) is a verb used in its proper meaning, id. ‘to put’ something/someone on/in/into etc., somewhere, and the second, おく of ておく is certainly the same word as the proper 置く, but it is used in this case as a sort of auxiliary verb in the form of -ておく, meaning ‘to leave something/someone in a certain situation as a result of action’. And, ておかない is simply the negative form of ておく. But, it seems to me that this negative ておかない is used less frequently than the affirmative ておく.
I can’t imagine a good example, but how about the sentence like this:
いやな経験は心にしまっておかないで、信頼できるひとに話すと気持ちが楽になるよ。[Don’t keep an unpleasant experience in mind. Tell it to someone you trust in, and you will feel relieved.]