Can I go to Serbia for 30 days with a US C1-D visa? Thank you, I have submitted this question more than ten times

Yes, quoted from the official website of the Serbian Embassy in China:

Holders of foreign passports with the valid Schengen visa, visa of the Great Britain, as well as the visas of other countries members of European Union, or with the valid visa of USA, can travel to the Republic of Serbia without a visa, and stay in Serbia 90 days in the period of 6 months. Furthermore, holders of foreign passports who have residence permit in the countries of Schengen zone, countries members of European Union, or USA can travel in the Republic of Serbia without a visa, and stay in Serbia 90 days in the period of 6 months.