[IB's answer and analysis]29. Which reaction has an entropy change closest to zero?
[IB's answer and analysis]
As you can see from Databook sec. 9 "Covalent bond lengths", the triple bond is shorter than the double bond is shorter than the single bond for nitrogen (and carbon). So the question boils down to recognizing that N2H4 is the only choice with a single bond between the nitrogen atoms.
When the keyword entropy appears on an IB test, you are probably being asked to notice if gas is consumed or produced by a reaction. Here the curveball is that you must find the reaction that does not produce a different amount of gas than it consumes. B is the only reaction with the same number of moles of gaseous compounds on both sides of the reaction.