“O Sir, content you’,
I follow him to serve my turn upon him.
We cannot all be masters, nor all masters cannot be truly
followed”.
The above lines were spoken by which of the following
characters?

The correct option is Iago.

These famous lines are spoken by Iago in Act 1, Scene 1 of William Shakespeare's tragedy Othello. He is speaking to Roderigo, explaining his duplicitous nature and his true motives for serving Othello.
In these lines, Iago admits that he only follows Othello ("him") to serve his own ends ("to serve my turn upon him") and explicitly states that not everyone can be a master, and not all masters are served with genuine loyalty. This serves as a key introduction to Iago's deceptive character and the themes of betrayal and appearance versus reality in the play.