Eliminate left recursion
S $\rightarrow$ Aa|b
A $\rightarrow$Ac|Sd|f
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$S \rightarrow Aa | b \\\\ A \rightarrow bdA'| fA' \\\\A | \rightarrow cA'| adA'| \epsilon$
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$S \rightarrow Aa | b \\\\ A \rightarrow cdA'| dA' \\\\ A' \rightarrow bA'| adA' |\epsilon $
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$S\rightarrow Aa|b \\\\ A\rightarrow bdA'|fA'$
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$S \rightarrow Aa| b$
$A \rightarrow bdA'|fA'$
$A'\rightarrow cA'|adA'$
A
Correct answer
Explanation
$A\rightarrow Ac|Sd|f \rightarrow Ac|Aad|bd|f $
Eliminating the immediate left recursion in A
$A \rightarrow bdA'| fA'$
$A| \rightarrow |adA' | \epsilon$
So the resulting grammar is option A.