The travel writer must invite ______ ; few travelogues, if any, have ever been inspired by a languorous afternoon poolside.

Hint lies in "languorous afternoon" - indicating laziness, metal/physical lethargy. Sentence structure with "if any" demands first blank be a word that is opposite to "langurous" - could be excited/energized

A. travail
- difficulties (can stimulate oneself) - lets park this
B. tribulations
similar in mean to A - so we have got a pair (yet lets look at other options - there is less likely to be a pair which will be so close in meaning, if there is a pair it would most likely be an opposite or something tangential to the concept)

C. excitement
- makes sense and matches with our guess - lets look for its partner

D. scandal
- doesnt make sense
E. tranquility
- peace, calmness (opposite to what we are excpecting)
F. serenity
As expected we have got a pair which is opposite in meaning to the blank i.e. E & F

We were unable to find a suitable partner for C - thus A & B makes sense