The correct answer is option 'D' - Dusk. Let's understand why.

- The phrase "That is why the window is kept open every evening" suggests that there is a specific reason for keeping the window open during the evening.
- The phrase "till it is quite" implies that the window is kept open until a certain condition is met, and this condition is related to the time of day.
- The options provided are: a) Dawn, b) Night time, c) Morning, and d) Dusk. We need to determine which option best fits the given context.

Let's analyze each option:

a) Dawn: Dawn refers to the time of day when the sun begins to rise, usually in the early morning. However, the phrase "till it is quite dawn" doesn't make sense in this context. Dawn is not typically associated with the evening, which is when the window is being kept open.

b) Night time: Night time refers to the period when it is dark outside, usually from sunset to sunrise. While it is true that evenings are part of the night, the phrase "till it is quite night time" sounds redundant. Additionally, keeping the window open during the entire night might not be practical or necessary.

c) Morning: Morning refers to the early part of the day, typically after sunrise. However, the phrase "till it is quite morning" doesn't fit the context of keeping the window open during the evening. Morning is a time when people usually wake up and start their day, which is different from the evening.

d) Dusk: Dusk refers to the period of partial darkness between sunset and nightfall. This option seems to be the most appropriate because the phrase "till it is quite dusk" indicates that the window is kept open until it is completely dark outside. This aligns with the idea of keeping the window open in the evening.

Therefore, the correct answer is option 'D' - Dusk.