Step-by-step explanation:
In the case of trans-decalin, there is no steric interaction between the rings because they are at their maximum distance. As a result, the trans conformation is more stable. Another reason is that trans-decalin does not undergo ring flipping. So, it remains rigid and is more stable.
Stearic interactions between the two rings result in a less stable conformation in case of cis-decalin.It undergoes ring flipping so, unstable conformation. (The First image)
Hence, trans-decalin is more stable than cis-decalin.(The second image)
Hopefully this helps! :)