Explanation:

The Mahaweli River is a 335 km (208 mi) long river, ranking as the longest river in Sri Lanka. It has a drainage basin of 10,448 km2 (4,034 sq mi), the largest in the country, which covers almost one-fifth of the total area of the island.The real creation of Mahaweli ganga starts at Polwathura(at Mahawila area), a remote village of Nuwara-Eliya District in bank Nawalapitiya of Kandy District by further joining of Hatton oya and Kotmale oya.The river reaches the Bay of Bengal on the southwestern side of Trincomalee Bay.The southern part of Sri Lanka is hilly, while the northern part consists of an extensive flat plain. This plain is drained by the Mahaweli Ganga, Yan and Aruvi rivers. Hence option 1st is correct.