After a copper smelter begins operation, local populations of plants downwind of the plant begin to adapt to the resulting air pollution. Scientists document, for example, that the acid tolerance of several plant species has increased significantly in the polluted area. This is an example of a response to:.
a) stabilizing selection.
b) heterozygote advantage.
c) disruptive selection.
d) genetic drift.
e) directional selection.

e) directional selection

Explanation:

Directional Selection:

* Another example is illustrated in the image. The graph shows the shifts in allele frequency.

Plant species that have increased their tolerance to acid in the presence of environmental contaminants is an example of a response to directional selection (option e).

Explanation:

Directional selection favors a specific trait within some individuals in the population, because they develop greater biological efficiency or have adapted better to environmental changes.

If a graph is made according to the characteristic of "acid tolerance" -where perhaps few plants are tolerant- the presence of a contaminated environment will favor the reproduction and survival of acid-tolerant plants, making resistant plants more frequent in the graph.

Because environmental conditions change, when the change persists over time, an inherited trait -such as acid resistance- is favored, making the population with this trait more frequent.

Learn more:

Differences between directional selection and disruptive selection link