a. What is the role/function of the phrase "in turn" in the sentence below? Preposition, conjunctions?
b. What is the best replacement word for "in turn" in lay terms for the sentence below?
example: As the case for most of the employee shares, he cannot exercise the options straight away, rather these options only vested and in turn became exercisable over a 4 year period after he became an employee.
Hi, Tony—"In turn" is not one word. It is two words. Together they form a phrase—a prepositional phrase, in which "in" is a preposition and "turn" a noun (the object of the preposition). The phrase has a rhetorical usage, which you are seeing in your example. Below is a definition of the phrase from The Oxford English Dictionary (OED). I have highlighted the especially relevant portion:
b. Phrases. (a) by turns (also †by turn), one after another in regular succession; successively, in rotation. (b) in turn, in turns, each in due succession: = (a). (in turn is also used rhetorically like in one's turn: see .) (c) in one's turn, in one's due order in the series. (Often also used rhetorically to indicate an act duly or naturally following a similar act on the part of another, but without the notion of pre-arranged succession.)