What word would complete this: Are you sure you’re telling me the truth? What do you mean _____ that?

The most suitable word to complete the sentence, "Are you sure you’re telling me the truth? What do you mean _____ that?" is "by." This usage is common in English when seeking clarification about what someone means. Here, "by" conveys the idea of asking for the reasoning or explanation behind the statement referenced.

In this context, asking "What do you mean by that?" indicates that the speaker is inquiring for further details or clarification regarding the truth of the statement made. It’s a way to engage in a deeper conversation about the intentions or meanings behind the words that were just spoken.

Alternative options, like "about," could imply a general inquiry into the subject but lack the specificity that "by" provides in asking for clarification. "For" and "on" would not fit the context grammatically or semantically, as they do not align with the common phrase used in English to seek deeper understanding.