Final answer:
The correct preposition to complete the sentence is 'para' which indicates the intended recipient or purpose.
Step-by-step explanation:
The correct preposition to complete the sentence is para.
The sentence translates to 'This souvenir that I bought in Santo Domingo is for my grandfather.'
In this context, para indicates the intended recipient or purpose, showing that the souvenir was specifically bought for the student's grandfather.
Learn more about Prepositions in Spanish