In the circuit shown, the resistance $r$ is a variable resistance. If for $r = fR,$  the heat generation in $r$ is maximum then the value of $f$ is 
  • A$\frac {1}{4}$
  • B$1$
  • C$\frac {1}{2}$
  • D$\frac {3}{4}$
JEE MAIN 2016, Diffcult
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