Sentence:  And in (121) below, we see that when a wh-NP is used as the Object of a <WSD>Preposition</WSD>, the whole Prepositional Phrase can undergo WH <small>MOVEMENT</small>:/  (121) (a)      ["To whom"] can I send this letter —?/  <font color=white>(121)</font> (b)      ["About what"] are they quarrelling —?/  <font color=white>(121)</font> (c)      ["In which book"] did you read about it —? 
Meaning of the word marked with <WSD> tags:  Any of a class of non-inflecting words typically employed to connect a following noun or a pronoun, in an adjectival or adverbial sense, with some other word: a particle used with a noun or pronoun (in English always in the objective case) to make a phrase limiting some other word.