For converting any base(say $n$) to decimal, we need to multiply the digits with their weights. And weights are equal to $n$ to the power place of the digit in the number, with respect to the radix point.
So, for converting $(0.4051)_8$ to decimal, we need to multiply the digits with their respective weights.
i.e.
$(0.4051)_8$
$= 0 \times 8^0 + 4 \times 8^{-1}+ 0 \times 8^{-2} + 5 \times 8^{-3} + 1 \times 8^{-4}$
$= 0.510009765625$
$\approx 0.5100098$
Hence option (A)