To determine which statement about diborane is not correct, let's analyze each option:$\\$• Option 1: $$There are two 3-center-2-electron bonds.$\\$This statement is correct. Diborane $(B_2H_6)$ has two 3-center-2-electron bonds, also known as banana bonds, involving two boron atoms and two bridging hydrogen atoms.$\\$• Option 2: $$The four terminal B-H bonds are two-center-two-electron bonds.$\\$This statement is correct. The four terminal hydrogen atoms form normal 2-center-2-electron bonds with boron atoms.$\\$• Option 3: $$The four terminal hydrogen atoms and the two boron atoms lie in one plane.$\\$This statement is correct. In diborane, the four terminal hydrogen atoms and the two boron atoms are indeed coplanar.$\\$• Option 4: $$Both boron atoms are $sp^2$ hybridized.$\\$This statement is incorrect. In diborane, each boron atom is $sp^3$ hybridized, not $sp^2.$ The $sp^3$ hybridization allows for the formation of the 3-center-2-electron bonds.$\\$Therefore, the incorrect statement is Option 4.