Valmiki Ramayana Ayodhya Kanda Sarga 8 Shloka 14
Original Shlokaधर्मज्ञो गुरुभिर्दान्तः कृतज्ञस्सत्यवाक्छुचिः। रामो राज्ञ स्सुतो ज्येष्ठो यौवराज्यमतोऽर्हति।।2.8.14।।
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राम: Rama, धर्मज्ञः knower of righteousness, गुरुभि: by elders, दान्त: selfrestrained, कृतज्ञ: grateful, सत्यवाक् truthful, शुचि: undefiled, राज्ञ: king's, ज्येष्ठ: eldest, सुत: son, अत: therefore, यौवराज्यम् as heirapparent, अर्हति is fit for.
Rama knows righteouness. He has been taught selfrestraint by elders. He is grateful and truthful. He is undefiled. He is the eldest son of the king. Therefore, he deserves to be the heirapparent (of the kingdom).