The correct option is
C in case
Option A - 'In order that' means 'so that'. For example, 'The staff must stay committed in order that the project succeeds.' As the sentence doesn't explain any reason, we can't use 'in order that' here. Thus option A is incorrect.
Option B - 'Unless' means 'except if'. For example, 'You can't go out unless you finish your work.' The given sentence explains a condition and not an exception, hence we can't use 'unless' here. Thus option B is incorrect.
Option D - 'While' is a conjunction which means 'during the action/at the same time'. For example, 'I read the book while you were bathing.' As the given sentence doesn't explain two actions happening at the same time, we can't use 'while' here. Thus option D is incorrect.
Option C - 'In case' means 'possibility of a situation'. For example, 'Bring a map in case you need it.' As the given sentence explains the possibility of 'rains', we can use 'in case' here. Thus option C is the correct answer.
'Take this umbrella with you, in case it rains.'