The first part provides the option of keeping the window open. The second part offers an alternative to that. Thus, it can be concluded that the second part of the sentence provides an option of closing it. Thus, the answer must mean 'close'.
Option A is incorrect. Get does not mean 'close' and thus, does not fit in the blank.
Option B is correct. Shut means 'to close'. Thus, it has the required meaning and is the best fit for the blank.
Option C is incorrect. Open is the exact opposite of 'close'. Thus, it cannot be the correct answer.
Option D is incorrect. Bring means 'to get'. It has no relation to the required meaning and does not fit in the blank.