The answer is B. It can reproduce quickly.
A single-celled organism, as its name explains, consists of only one cell. This cell must be small because, in a large cell, it would be needed a great time for information, nutrients and other molecules to travel from one part of the cell to another (choice A. is eliminated). That small cells are able to reproduce quickly because their dimension allows them to divide to new cells quickly (even 20 minutes). Still, although it is only one cell it must maintain its structure (choice C is eliminated), and cannot tolerate all environmental conditions (choice D is eliminated).