While talking about her poor marks in mathematics Avi says "I just don't have the sense for numbers" Avi is attributing his performance to:

Attribution theory explains how individuals interpret and assign causes to their successes and failures. These causes can be internal (within oneself) or external (outside oneself), stable (unchangeable) or unstable (changeable), and controllable or uncontrollable.

Key Points

• In this case, Avi is making an internal attribution, meaning he believes the cause of his failure lies within himself.
• Avi believes his difficulty with mathematics is due to a lack of natural talent or inherent skill, which he perceives as unchangeable.
• This belief is often associated with attributing failure to an internal, stable factor, specifically, a deficiency in ability.

Hence, we can conclude that Avi is attributing his performance to a lack of ability.