Option 1 -> crystallise means to make clear or definite; doesn't fit the context of addressing pollution causes.
Option 2 -> mitigate means to reduce or alleviate the severity of something; appropriate for lessening pollution causes.
Option 3 -> expedite means to speed up or accelerate; contradictory as you wouldn't want to hasten pollution causes.
Option 4 -> perpetrate means to commit or carry out (typically harmful acts); implies causing harm rather than preventing it.
Hence, Option 2: mitigate -> Government and civil society working together to reduce or lessen the causes of pollution is the logical and grammatically correct choice. Mitigation involves taking action to minimize environmental damage. -> correct