Because she had ADHD.
题意:
为什么塔尼斯不能一次认真学习一个多小时呢?
细节理解题。根据第二段,第二、三句The fact was that I was found to have ADHD(注意力缺陷多动障碍). I felt worried all the time and I was unable to study carefully for more than an hour at a
time.事实是,我被发现患有多动症。我一直很担心,我不能认真学习超过一个小时。可知,塔尼斯不能一次认真学习一个多小时的原因是她患有多动症。故答Because she had ADHD.