[susan53]

2) The manager is said to have arrived back from Paris where he ________________ some European partners.

Options:
a) Would meet b) Had met

Why did they opt for past perfect tense and not past simple when the manager "has arrived", not "arrived"?

Just "met" was not offered as a correct answer. Can it be used?

Yes "met" would be fine. Had met is possible because the two events happened in sequence, and the past perfect clarifies which one happened first. But as this is clear from the context (we know the manager arrived back, "where" situates the meeting in Paris, and therefore the meeting must have happened before the arrival), it's not obligatory - the simple past can also be used.

English grammar in general doesn't like doing things twice - so for example, when the sequence is indicated by an adverbial (eg before/after) then it's not necessary for the verbs to do the work of sequencing. I have the choice of saying eg Add the tomatoes after you have fried the onions or simply Add the tomatoes after you fry the onions; The plane had left before we arrived at the airport or The plane left before we arrived at the airport. The adverbial (or in other cases the context, as in your example) makes the sequence clear and sequencing by the verbs is unimportant and therefore optional.

But sequence is not always clear from the adverb/context. For example, if we use when in my sentences above, there's an ambiguity. When has two meanings - at the same time and after. So now the verb becomes essential to disambiguate the sentence. Add the tomatoes when you have fried the onions has a different meaning to Add the tomatoes when you fry the onions; The plane had left when we arrived at the airport is different from The plane left when we arrived at the airport. If the perfect is used, that event happens before the other; if the simple is used the two events happen at the same time.