Complete with ser.

Puerto Rico y la Republica Dominicana _____ dos islas en el mar Caribe.

son

Puerto Rico y la República Dominicana son dos islas en el mar Caribe.

Complete with ser:

Puerto Rico y la República Dominicana son dos islas en el mar Caribe.

To complete the sentence, we use the verb "ser" in its third-person plural conjugation "son" since we are talking about Puerto Rico and the Dominican Republic being two islands. "Son" is the correct form of "ser" to match the subject "Puerto Rico y la República Dominicana". Therefore, the complete sentence is "Puerto Rico y la República Dominicana son dos islas en el mar Caribe".

You need to use the third person, plural, indicative present tense.

http://www.123teachme.com/spanish_verb_conjugation/ser