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

A The importance of eloquent and lengthy prayers. B The necessity of offering material gifts alongside prayers. C The assurance that God will answer prayers immediately and exactly as requested. D The need for unwavering faith and perseverance in prayer, even when answers are delayed.