die Mitternacht

The word 'Mitternacht' is feminine, so it takes the feminine dative demonstrative pronoun 'dieser' (or 'diese' in the accusative case, but here it's dative after 'um'). However, 'um Mitternacht' is a fixed expression and usually does not take an article or demonstrative pronoun unless specified. In this specific fairy tale context where it is meant to be 'at this midnight', 'diese' would be used to refer to 'this specific midnight'. Given the options, 'diese' is the most fitting for 'this midnight' in a narrative context. But in everyday German, 'um Mitternacht' simply means 'at midnight'. If we are strictly following standard grammar, no demonstrative pronoun would be used here. For the purpose of this C1 exercise testing demonstratives, 'diese' is the intended answer for 'this midnight'. If the question implied a general midnight, the options would be different or it would just be 'um Mitternacht'.

The adjective 'spät' (late) is often used with 'Mitternacht' to emphasize that it was well past midnight. 'Dunklen' (dark) is also possible but less common in this specific context.

The idiom 'bis tief in die Mitternacht' means 'deep into the night/midnight' and implies working until very late. 'Spät' (late) is an adverb, not an adjective that would fit directly here. 'Weit' (far) could work if it was 'weit nach Mitternacht' (well after midnight) but not 'weit in die Mitternacht'. 'Lang' (long) doesn't fit this idiomatic expression.

'Voller Mitternacht' is an idiomatic expression that can imply a sense of the dead of night or the full extent of midnight, often used in a somewhat poetic or atmospheric context.

'Volle Mitternacht' is a common expression meaning 'exactly midnight' or 'the full midnight' when referring to the exact time. 'Schlagenden' (striking) could refer to the clocks striking, but 'vollen' describes the time itself. 'Exakten' (exact) is grammatically correct but less idiomatic in this context.

'Festgelegten' (fixed, determined) fits best here to describe the 'official' or established midnight, implying a contrast with the actual time they left. 'Bestimmten' (certain, specific) could also work, but 'festgelegten' often carries a stronger sense of a set standard.