<p><span style="color:#27ae60;"><span style="font-size:20px;">Solving the Problem</span></span></p>
<p>To solve this problem, we use the <strong>Percent Greater Formula</strong>:</p>
<p style="text-align: center;">$$\frac{larger \: number - smaller \: number}{smaller \: number}$$</p>
<p>Airline B's average rating for convenience is $8.0$ and Airline A's average rating for convenience is $5.1$. Let's plug these values into the formula:</p>
<p style="text-align: center;">$$\frac{8.0 - 5.1}{5.1} = \frac{2.9}{5.1} \approx 57 \%$$</p>
<p>Thus, <strong><span style="color:#27ae60;">the correct answer is D</span></strong>.</p>