Prepositional phrases are a group of words containing preposition which lacks either a verb or a subject. Such phrases always have two basic parts: object and preposition. Prepositions are mostly followed by objects.
Using option B will justify all the above rules. 'On behalf of' is the preposition and 'employees' is the object. 'On behalf of my employees' is a prepositional phrase.
Options A, C, and D are not creating prepositional phrases with object 'employees'.
Thus, option B is the correct answer.