<span>My best friend and I enjoy wandering through the catacombs.
If the underlined verbal phrase is <em>wandering through the catacombs, </em>then it serves the role of object of the verb.
What do my friend and I enjoy? Wandering through the catacombs.
</span>
A all of the nouns. It could have been d, but if the quote is embedded, capitalizing the first letter would be incorrect grammar.