At the Harappan site of Kalibangan, fire altars have been found. Fire altars are elevated structures used for religious rituals and ceremonies involving the worship of fire. These altars were an important aspect of the religious practices of the Harappan civilization.

Option 1, Kalibangan, is the correct answer because fire altars have been discovered at this site during archaeological excavations. This discovery indicates the presence of religious ceremonies and rituals involving fire worship at Kalibangan.

Option 2, Harappa, does not have any mention of fire altars in the archaeological findings. Therefore, fire altars are not found at Harappa.

Option 3, Mohenjo-daro, also does not have any evidence or mention of fire altars being found. Therefore, fire altars are not found at Mohenjo-daro either.

Option 4, Rakhi Garhi, does not have any information available regarding the discovery of fire altars. Therefore, fire altars are not found at Rakhi Garhi.

In conclusion, fire altars are specifically found at the Harappan site of Kalibangan, making option 1 the correct answer.