If you type that exact phrase into Google or Bing, you will find dozens of DLL download sites (DLL-files.com, DLLme.com, Fix4DLL.com, etc.). Here’s why you should avoid them:
If you own the game on Steam, use the built-in tool to redownload only the missing file: Steam Library Right-click Call of Duty: Black Ops II and select Properties Local Files (or Installed Files) tab. i--- Download Steam Api.dll For Call Of Duty Black Ops 2