Which is more stable, a trans-1,4-disubstitutedcyclohexane or
its cis isomer?

If the substituents are the same, the cis distribution is more stable than trans distribution.

Explanation:

A cis cyclohexane is one in which both substituents are oriented towards the same face of the ring regardless of the conformation.

A trans cyclohexane has substituents on opposite sides of the ring.

In trans-1,4-disubstituted cyclohexane, the conformation with the two substituent groups in equatorial is more stable than the constitution that has the axial groups.

In cis-1,4-disubstituted cyclohexane, both conformations have the same stability as they have an axial and an equatorial substituent, so they have the same energy and there is no equilibrium shift.