Explanation

The sentence says that people who work in the service industry put on a smile for the customer, implying that they hide their true emotions in some way. Masquerade is close, but it means either “to pretend to be someone” or “to be disguised as something else.”

Neither of these are quite right, so the correct word is mask. For the second blank, you need something like a requirement. Apex is a high point (i.e., “The apex of my career was playing for the Knicks.”). Conceit means a fanciful notion. Part and parcel, meaning “an essential piece of something,” is the correct phrase.

Mask, part and parcel.