The potential difference across the source will be c)12V
What is Ohm's law ?
Ohm's law states that  the current through a conductor between two points is directly proportional to the voltage across the two points .
since , R(A) , R(B) AND R(C) are in parallel combination , hence  equivalent 
resistance of the circuit will be 
1/R = 1/R(A) + 1/ R(B) + 1/ R(C) 
1/R= 1/2 + 1/4 +1/8 
1/R = 7/8
R = 8/7 ohm
from ohm's law 
Voltage = current * resistance 
V( source ) = current (in circuit ) * R (equivalent resistance)
V = 10.5 * (8/7)
V = 12 V 
The potential difference across the source will be c) 12V
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