This is a **line** of code.
How can I make line
and only line
shown in bold font?
You can't and there is really no need to. If you want to "highlight" something in your code, point it out in a code comment.
This cannot be done using markdown. Your only option is to resort to literal HTML:
<pre><code>This is a <strong>line</strong> of code.</code></pre>
Which renders as:
This is a line of code.
But this is ugly and hackish, and mostly unnecessary. Do like Bart said and use a comment to call out important things in your code samples instead.