Generally, the best place to put "just" is immediately before the word you want to emphasise.
When this word is a main verb, and the verb is a phrase like "is sleeping", then it goes before the participle.
In your examples, a. and c. sound natural and obey this rule.
If you place the "just" (and other adverbs like "only" and "especially") before a word which is different to the one you want to emphasise, it sometimes creates ambiguity, and often feels odd.