a. More N2O4 is produced.
As all species are gases, over more pressure, the side of the reaction with fewer gas molecules will be favoured as it is the one that occupies less space, where more molecules would fit in lesser space. N₂O₄ has the same number of atoms of 2NO₂, but the difference is that they are only in one molecule - occupy less space because all the atoms are closer to each other.
