The correct option is B provided that
The speaker tells the listener that they would lend their car as long as the person borrowing it (listener) keeps it safe. The sentence is condition-related, as the speaker would lend their car on the condition that the listener keeps it safe. The subordinate clause "you keep it safe" depends on the main clause "You can take my car" for its full meaning, and the former is the condition depending on which the action in the main clause (lending the car) will take place. From the options, 'provided that' is used for condition-related sentences, it means 'if, and only if'. Hence option C is correct. 'So that' is used in purpose-related sentences, where one clause is the purpose or reason and the other is its result. The given sentence does not state any purpose, the speaker does not lend his/her car for the reason that the listener keeps it safe, hence option A is incorrect. Similarly, 'in order that' is also used for purpose-related sentences, and D is incorrect. 'Even if' is used in a supposition or an imaginary situation, where the subordinate clause expresses an imaginary situation, but here "you keep it safe" is not hypothetical, but the condition based on which the person lends his car. Hence option B is incorrect.