Why can't the production of H2S2O7 be in high pressure conditions?

Its B or C :

The actual answer I believe is that it is not economical to use high pressure - it costs a lot more money. The yield increases but not significantly enough to compensate for this. So they use normal pressure because it is enough for industrial production - but they would use higher pressure although the disadvantages (money) outweigh the advantages (increased yield)l.

The whole contact process for sulphuric acid manufacture is carried out at atmospheric conditions

The SO2 / O2 gas stream is passed through the V2O5 catalyst beds in extremely large reactor chambers:
The reaction
2SO2(g) + O2(g) ↔ 2SO3(g)
should, according to le Catellier be favoured by an increase in pressure, in practice this is found unnecessary. The stream is at atmospheric presssure and the final step, the dissolving of the SO3 into H2SO4 to produce H2S2O7 is easily accomplished and pressure is not required - the SO3 forms a compound with the H2SO4, and so increased pressure is not required. If the SO3 was merely dissolved in the liquid H2SO4, there could be same argument in favour of an increased pressure in accordance with Henry's Law, but as an intermediate compound is formed, this is unnecessary.
The answer is b)