Smirk is the following



Soooo

Indeed, it is a source of common (i) __________ among engineers,

In the end the fact that the layman does not be able to make a distinction is a source of fun for them or amusement

Basically they laugh because they know something that others do not

I hope now is more clear

i marked C and D, since i knew what parlance meant, but after checking colloquialisms meaning, in ordinary conversation, i feel E is a better fit. since question suggests that people in particular profession (parlance) know both are different, common people (colloquialism) think both are same.

Indeed, it is a source of common (i) ___AMUSEMENT_______ among engineers, who smirk that the average layman cannot differentiate between the man who runs a locomotive and the man who designs a locomotive. In ordinary (ii) _____PARLANCE_____, both are called engineers.

That engineers “smirk” suggests that they feel amusement in the first blank. Parlance is the best choice for the second blank because it means that the language of people in a particular profession.

Indeed, it is a source of common (i) __________ among engineers, who smirk that the average layman cannot differentiate between the man who runs a locomotive and the man who designs a locomotive. In ordinary (ii) __________, both are called engineers.

To smirk is to smile in an irritating and smug fashion, therefore, the word for the first blank is amusement.

Since, the engineers are amused by the fact that the average layman cannot differentiate between the man who runs a locomotive and one designs it, the second sentence should convey the sense of:

In ordinary language, both are called engineers.

Therefore, the word for the second blank is parlance, which is a particular way of speaking or using words, especially a way common to those with a particular job or interest.