Good point. The detail seems unclear. So, what are the possibilities? He could have caught the ball with...
- left foot on the floor, right in the air
- right foot on the floor, left in the air
- both feet on the floor
- neither foot on the floor
In the first case, the left would be the pivot foot.
In the second, the right would be. However, the way it's worded makes this case seem less likely to me. He would be stepping forward with his left when catching the ball. (which is not what it says) Then stepping first with the right then the left. That leaves him covering a lot of ground and would be more obviously a travel than the other possibilities. I would discount this possibility.
If it were both feet, the left would be the pivot.
We don't have enough information to discuss the final option. It seems least likely to me, so I would discount it as well.
That leaves two plausible options, both of which would make the left foot the pivot foot. At least that's the way I read the question. Am I missing something?