M. N. Roy
To identify which thinker is attributed to the statement, "So long as the millions live in hunger and ignorance, I hold every man a traitor, who having been educated at their expense, pays not the least heed to them," we need to analyze the philosophies and teachings of the given options.
Therefore, the correct answer is Swami Vivekananda. He emphasized the duty of educated individuals towards the marginalized and spoke about the betrayal of not helping those in need, aligning perfectly with the given statement.