What is the best vitamin to give my six year old daughter to concentrate?

Healthy diet is #1: Usually, if you are able to give a varied diet with fruits and vegetables, starches, iron rich foods, and dairy, vitamins may not be necessary. Otherwise, most vitamins are labeled with % of minimum daily requirements and are fairly equivalent from one brand to the next. Children at 6 may prefer a chewable tablet or gummy type vitamin.

See below: No specific vitamin is known to boost concentration. Low iron count (anemia) can adversely affect concentration though.