In the Parable of the Persistent Widow (Luke 18:1-8), what key aspect of prayer is Jesus emphasizing through the widow's relentless appeals to the unjust judge?

A God is unjust and requires constant nagging to grant requests. B Effective prayer involves manipulating God through emotional appeals. C The primary purpose of prayer is to change God's mind. D Believers should persevere in prayer with unwavering faith and persistence, trusting that God will ultimately bring justice, even when it seems delayed.