At room temperature, the eclipsed and staggered forms of ethane cannot be isolated because (AFMC 2010)

Staggered conformation of ethane is the most stable conformation of ethane and eclipsed conformation is the most unstable one. The energy difference between staggered and eclipsed ethane is 12.55 1 KJmol. The cis and trans isomers can be separated if there would have been large difference in the energy level. This energy is not large enough to prevent rotation at room temperature. Thus, the correct option is (a) they reconvert rapidly.