Which UART is used on the Explorer 16 development board with PIC32?
The UART1 is used.

But beware, the Explorer 16 board uses two 74HCT4053 analog multiplexers to simplify the con-
nections between itself and any daughter boards. U6 and U7 provide active control of 
the cross-wire capability on SPI1 and UART1, with a hardware flow control signal 
provided by three I/O pins.