The correct answer is:
His tutor did not have time to give him private lessons so he was encouraged to study on his own.
Step-by-step explanation:
Quoting Euler himself in his unpublished, autobiographical writings, his tutor was named Johann Bernoulli.
Euler states "...True, he was very busy and so refused flatly to give me private lessons; but he gave me much more valuable advice to start reading more difficult mathematical books on my own and to study them as diligently as I could; if I came across some obstacle or difficulty, I was given permission to visit him freely every Sunday afternoon and he kindly explained to me everything I could not understand..."