Which isomer has the lower energy and which is fiexible in cis and trans decalin ?

Explanation

In trans-decalin, only the rigid (e, e) conformer is possible. Flipping to (a, a) orientation is impossible, because the diaxial bonds point `180^(@)` away from each other and cannot be bridged by only four C atoms to complete the second ring. In cis-conformation, fusion is conformer. Therefore, cis-form is flexible. The trans conformer (e, e) has lower energy than cis conformer (e, a).