The question in this context asks us to suggest a suitable rewrite for the underlined portion of the sentence . That portion is numbered 29 and I've indicated it through an arrow. On the right you've to choose between four alternatives, the first option (not shown in picture) being "NO CHANGE" - indicating the current sentence as it stands is correct. The other three options are shown in the image.
I incorrectly chose C; while the machine says B is correct.
According to what english I've learnt, the sentence "Take Bartlett pears for instance, unless ... time, they will..." creates a continuous tone of words (Take Bartlett pears for instance) - as I would speak it naturally - before the first comma, then separates off the "unless" block between two commas, then carries on the last phrase. This appears to be a more continuous and natural structure to me.
The option B - indicated as correct - results in "Take Bartlett pears, for instance: unless ... time, they will..." - which not only creates a pause midway through an incomplete phrase (There's a sudden pause after "Take Bartlett pears"), but also surrounds the "unless" block with a colon on the left and a comma on the right, which should not be correct imo.
I hope I've been as detailed as possible in explaining my problems and I hope someone can give back an as detailed answer explaining what exactly is/are wrong in my train of thought.