Steric hindrance is observedX = cis - 2 - butene & Y = trans - 2 - butene

→ In cis-2-butene two bulky methyl groups are on same side, hence in between them strong spatial(steric) repulsions are operating,hence in cis-2-butene strong steric hindrance is observed.→ Where as in trans-2-butene the two bulky methyl groups are on opposite side and hence there is no steric repulsions,hence steric hindrance is absent.