The correct option is C had just left
The sentence states two actions such that one takes place immediately after another. Also, these actions have taken place in the past, as implied by the verb "entered" (and 'just left').
The past perfect tense is used to indicate that an action was completed in the past before another action commenced or took place.
This is quite similar to the given sentence, where one event 'the guests (left)' takes place before another 'I entered...house'. Hence the underlined verb form must be in the past perfect tense. (Past perfect means past of the past.) 'Just left' is not in the required tense form. Hence it must be changed.
Option C: 'Had just left' is in the past perfect tense form. Hence this is the correct option.
Option A: 'Just leave' is in the simple present tense, but the given sentence is in the past. Hence A is incorrect.
Option B: 'Have just left' is in the present perfect tense. It is used when an action has completed recently, but it does not relate to the second part of the sentence "when I entered ...". Hence B is incorrect.
Option D: 'Had just been left' is in the passive voice whereas the sentence is in active voice. Hence D is incorrect.