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The value of magnetic field given is not clear... 10-8 ??  is that  10^{-8}

The magnetic field B of induction at a point P at distance d from the long wire is given by :  B =  Mu_0  i / (2 π d)

   It is inversely proportional to the distance.

   So the field at 12 cm will be = 1/3 of field at 4 cm.. which is 10^{-9} Tesla.
  So   (A)  3.33 * 10^{-8} Tesla

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