Groups of people who care about the ... spend

The sentence is incomplete. To make a complete sentence, you need to choose between 'environmental' and 'environment'.

If you are talking about a group of people, the adjective 'environmental' would modify the noun 'groups'. For example, 'environmental groups'.

If you are talking about what the people care about, the noun 'environment' would be used. For example, 'Groups of people who care about the environment spend...'.

Based on the structure of the sentence provided, the word 'environment' is the more logical choice to complete the phrase.

Answer: environment