Editing the style associated with a survey will let you modify the look of the text for the entire question or message, but not individual portions of the message.
To change specific portions, you would need to wrap the words you wish to change the appearance of in the appropriate HTML codes, such as the following:
HTML Code:
<b>Bold</b>
<i>Italic</i>
<u>Underline</u>
e.g. What is the <i>
exact reason</i> for your visit to <u>
SuperCorp</u> today?