Question

read the passage and then answer the question.
auxins are plant hormones that promote cell growth and
elongation, and they are also among the principal hormones
responsible for plant tropism. for example, auxins in plant
cells cause plant shoots to grow towards sunlight. they do so,
according to one theory, because sunlight degrades the
hormones, leading to a higher concentration of auxins on the
shade - facing side of the plant. as a result, that side grows more
quickly, thus bending the stalk towards the sun. ironically, while
these hormones are produced by plants, they can also be used
to make herbicides because some plants are sensitive to high
auxin levels.
what is the meaning of tropism as used in the passage?
the change of auxin concentrations in a plant
directional movement of an organism in response to
an external stimulus
the process by which a plant converts sunlight into
chemical energy
the growth of plant parts, such as shoots, leaves,
and roots

Explanation:

The passage gives an example of tropism: plant shoots growing towards sunlight. This shows that tropism is a directional movement (towards sunlight) in response to an external stimulus (sunlight). The first option is about auxin concentration change (not the meaning of tropism). The third option is about photosynthesis (wrong). The fourth option is about general plant growth (not specific to the response - based directional movement as in the example).

Answer:

directional movement of an organism in response to an external stimulus