The correct option is
C By / on
The first given sentence indicates a means of communication.
The preposition 'by' is used to show means of communication.
We can thus eliminate options A and C as incorrect answers.
The second given sentence indicates by means of foot.
On is used to indicate place,specific date, direction, by means of or about.
With is used to indicate means, possession, contents, relationship, for and against.
Since on fulfills the meaning of the sentence,option B is the correct answer.