The infinitive or infinitive phrase can indeed be the subject of the verb. So we cannot reject d) on simple grounds.
To swim the English Channel is her dream.
To succeed requires diligence.
To travel from New York to Washington by train takes several hours.
However, the infinitive refers to the idea of the action, an action that could possibly occur, but it does not refer to an instance of the action happening in finite, real time.
The verb phrase 'is the source of revenue' wants as subject a form that expresses an action actually taking place in time, not a form that expresses the idea of the action outside of time. Revenue is earned by the activity, not by the timeless idea of the activity. Publishing refers to the ongoing activity, the action happening in time.
To eat is how we stay alive. ungrammatical
Eating is how we stay alive. grammatical
P.S. But with "why" (reason/intention) we want the form that is outside time:
To visit Santa Claus is why we went to the North Pole.
Visiting Santa Claus is why we went to the North
P.P.S. Things we do versus things we have in mind.