Hi
A : Das
explanation :
Mädchen ( ie : the young girl) is the subject of the sentence. so it cannot be "den" which does not exist for subject
With "junge " in singular form, ( plural would be "jungen ")
we know there is only one . so it cannot be "die " as Mädchen is neutral ( you must know it by heart or check in a dictionary) So it can not be " der " and has to be "das".