The formula you need is
TA = IA + (IA - FE)*ISADEV/°K)
TA=True Altitude
IA=Indicated Altitude = 10,000
FE=Field elevation=2,500
ISADEV=ISA deviation=-15
°K=Temperature in °C at IA +273=253
Inserting your figures in the formula gives
TA=10,000 + (7500*(-15)/253) = 9555
So answer B is correct