The answer is:
Switzerland
Because:
Campione d'Italia is an Italian exclave which is an enclave in Switzerland
Baarle-Hertog is a Belgian exclave which is an enclave in the Netherlands
Büsingen am Hochrhein is a German exclave which is an enclave in Switzerland
Nahwa is an exclave of the United Arab Emirates which is an enclave in Oman
San'kovo-Medvezh'e is a Russian exclave which is an enclave in Belarus
Here's more: