how to enable i2c for beaglebone rev c.

need help…anyone can answer
cannot detect address device thermal …

Insufficient information.

What version OS are you running?

If you are running one of the recent Debian releases, I2C-2, (in early software was called I2C-1) which is the one that comes out to detect other capes, on P9-17 and P9-18 is already enabled, and ready to use.

There are four (EEPROM memory) addresses that are reserved, but everything else is available to you to use.

What are you trying to do?

— Graham

cannot detect address device thermal …

What do you mean by “device thermal”…?

Like the Grove temperature sensor? Or the Grove temperature & humidity sensor? If so, you need the ADC grove as well. Like this: