Final answer:
The correct verb form is 'hagan' because the subject is plural.
Step-by-step explanation:
The correct verb form to complete the sentence is hagan. In this case, we need the third person plural form of the verb hacer (to do/make) because the subject las autoridades is plural. This is indicated by the plural pronoun hagan. The correct sentence would be Es indispensable que las autoridades hagan todo lo necesario para proteger a los animales.
Learn more about Verb forms in Spanish