I am doing a project on heat island effect using MOD11_A2 data. After processing in MRT I have separate bands. While importing them into ENVI, which bands should I select in the RGB option?
The first 10 scientific datasets in a MOD11_A2 file are the same as in MOD11_A1, described for example here. The first band contains land surface temperatures, and beyond that you get quality control flags, emissivities in two different bands, various geometric and time data fields. It doesn't make much immediate sense to construct a (false color) RGB from them. So the question would be, what is the goal of your visualization?
If you would like to get a true color RBG of your area, look into the MOD09 (or MYD09) product. You can combine bands 1, 4, and 3 to get a true-color RGB image.