Hi George —
C is certainly a tempting answer — the difference has to do with planning for a contingency vs trying to prevent that contingency from happening —
Imagine you were throwing a party, and you order pizza which they tell you will arrive in about 30 mins —
a) prepare for the possibility (contingency) that the pizza might be a bit earlier or later
b) work to try and ensure that the pizza restaurant does indeed deliver in about 30 mins —
Those are slightly different goals, and, in 29.1.19, the issue discussed is planning for contingencies, as opposed to planning to prevent them, and so (A) is a better answer than (C). The principle underlying is not that the manager is to blame because he was responsible for making sure the contractor finished on time, but rather because he should have planned for the possibility that the contractor might not.
HTH — MK