💡 Detailed Explanation
"～ものを" is similar to "～のに" and expresses dissatisfaction or regret that something did not turn out as expected. In this context, it conveys the feeling of "I could have helped, but it's a shame that I couldn't," making "ものを" the most appropriate choice. "ものから" indicates a reason/cause, "ものなら" indicates a hypothetical situation, and "ものか" expresses a strong denial, none of which fit the context.