You can borrow my laptop __________ good care of it.

The most appropriate choice to fill in the blank is "as long as you take." This phrase effectively expresses a condition under which the speaker is willing to lend the laptop. It implies that the borrowing is contingent upon the borrower taking good care of the laptop, and it has a more informal tone which suits everyday conversations.

The other options, while they also imply conditions, convey slightly different meanings or levels of formality. "Provided you take" is also a conditional phrase, but it sounds more formal. "Even if you take" suggests a negation, implying that taking good care wouldn't matter, which contradicts the intended meaning. "Assuming you take" implies an expectation without emphasizing the importance of taking care of the laptop, making it less suitable for this specific context.