My educated guesses would be the alternative C or D.
In a matter of days, to my thinking, has a ring of "shortly/soon (in a few days), it has a certain "prediction" quality to it. Whereas something that happened in a "limited" time would be something that happened "within days" (then within a matter of days as well).
Something that can be supposed (predicted/expected) to happen in a very limited ammount of time (in response to a crisis, for example) could also be "within" as in:
House Speaker John Boehner (R-OH) said Thursday that he expects the White House to send Congress a new authorization for a use of military force agreement within a "matter of days." (NBC News)
(dentro de alguns dias/em alguns dias...)
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Here we see that "you could have your garden transformed within a matter of days" means that the fact in the realm of possibility (and of a limited time).
Anyway, both (within AND in) seems like being interchangeable in this case. Anyway, for guessing´s sake and if I had to choose I would tick the alternative D (perhaps just a matter of personal style here). But I admit that I could be wrong, it´s a hunch of sorts.