about the usage of articles

[sariputra]

Dear Sir / Ma'am

When I refer to the English Grammar Glossary of UsingEnglish.com
to find out the exact meaning of the word of preposition, I can find
such an explanation as follow :

A prepositional phrase is 'the' combination of a preposition and its
complement

Would you kindly let me know which is more advisable between
'the' combination and 'a' combination

and explain the reason.

I look forward to receiving your reply soon.

[Mister Micawber]

.
Either is fine, Sariputra. 'The' anticipates the specificity of the defining phrase 'of a preposition and its complement. 'A' would result from the first appearance of 'combination' in the sentence being produced.

It is primarily an arbitrary choice by the speaker, who may be ('a/an') writing linearly or ('the') thinking ahead.
.