Your answer, B, is correct. Nonvascular plants tend to grow small and close to the ground because they lack structures that allow them to grow large.
To arrive at this answer, you can use a process of elimination:
Option A states that nonvascular plants get less food than vascular plants. While it is true that nonvascular plants may have limited access to water and nutrients, this alone does not explain their small and low-growing nature. So, option A can be eliminated.
Option C suggests that nonvascular plants grow close to the ground because they have small stems. While nonvascular plants may have small stems, this is not the primary reason for their low-growing habit. Eliminate option C.
Option D suggests that nonvascular plants grow close to the ground in order to capture more sunlight. While nonvascular plants do rely on sunlight for photosynthesis, their low-growing habit is not specifically for capturing more sunlight. Therefore, eliminate option D.
By process of elimination, we are left with option B, which correctly explains that nonvascular plants grow small and close to the ground because they lack structures that allow them to grow large.