Fill in the blank with the most appropriate word.
We've finished _____ the house.
Option B is the correct answer as verb 'finished' needs an object to complete its sense and gerund 'decorating' is serving that purpose. Also, the sentence talks about the activity that happened in the past. So, a gerund, not an infinitive, can convey this sense best. This is why Option D cannot be used as it is an infinitive. Options A and C cannot be used as this sentence does not require a verb to complete its meaning.