According to the rational root theorem, -7/8 is a potential rational root of which function?

The complete question in the attached figure

we know that
   
The Rational Root Theorem establishes that if the polynomial 
P(x) = a n x ^(n) + a n – 1 x ^(n – 1) + ... + a2 x² + a1 x + a 0 
has any rational roots, then they must be of the form 
+/-{factors a0/factors an}

then 
case A) an=24   a0=-28
The number  24  has factors of +/-{24,12,8,6,4,3,2,1}
The number  -28  has factors of +/-{28,14,7,4,2,1}
in this case -7/8 is a potential rational root     

case B) an=28   a0=-24
The number  28  has factors of +/-{28,14,7,4,2,1}
The number  -24  has factors of +/-{24,12,8,6,4,3,2,1}
in this case -7/8 is not a potential rational root    

case C) an=30   a0=-56
The number  30  has factors of +/-{30,15,10,6,5,3,2,1}
The number  -56  has factors of +/-{56,28,14,8,7,4,2,1}
in this case -7/8 is not a potential rational root    

case D) an=56   a0=-30
The number  56  has factors of +/-{56,28,14,8,7,4,2,1}
The number  -30  has factors of +/-{30,15,10,6,5,3,2,1}
in this case -7/8 is not a potential rational root    

the answer is 
   -7/8 is a potential rational root of
f(x) = 24x7 + 3x6 + 4x3 – x – 28