Pronome Relativo "QUE" ( Relative Pronoun)

The most versatile pronoun is QUE ,which can refer to a person or thing, subject or object, translating "who(m)", "which" or "that". Note that unlike English the relative pronoun can never be omitted in Portuguese:
Examples:
O homem que eu vi
the man ( who/that) I saw

Qual é o onibus que vai ao centro?
Which is the bus that goes downtown?

The relative pronoun "Que" can be preceded by by the simple prepositions  a, com , de, em  and por  ( but not when these form part of a compound preposition ).
Examples:
A moça com quem eu falava
The girl ( who/that) I was talking to

Foi uma decisão de que ele se arrependeria
It was a decision (which/that) he would regret

ref.: Modern Brazilian Portuguese Grammar- A Practical Guide