What are some of the challenges that travel agents face in their careers today?

the correct answer would be b, when, queen, calder hall and london.

explanation:

we always capitalize words when a paragraph or sentence is starting, when an important name is used, when a place is being written, or simply when a noun is used, etc. so in this sentence, sentence is starting from 'when', so it will be written in capital letters. secondly, queen is considered to be an important person, so it will also be written in capital letter. next, the name of the town is mentioned, i-e calder hall, which will also be written in capital letters. in the last,   city name is being used, 'london', which will also be written in capital letters. so option b is the most appropriate one.