Final answer:
The appropriate verb to fill in the blank is 'Se prohibe' which means 'It is prohibited'.
Step-by-step explanation:
The appropriate verb to fill in the blank in the sentence 'hablar en la biblioteca' is Se prohibe. This translates to 'It is prohibited to speak in the library' in English.
Se prohibe is a reflexive verb construction in Spanish that is commonly used to express prohibitions or rules. It indicates that speaking in the library is not allowed.
Other options, such as 'Hay que,' 'Es necesario,' and 'Se debe' do not convey the same meaning of prohibition or restriction.
Learn more about Spanish verbs