The correct answer is option 1, Irrawaddy River. The Irrawaddy River is the longest river in Myanmar. It flows from north to south and runs through the country for approximately 2,170 kilometers. The river is of great importance to Myanmar, as it supports the agricultural and fishing industries and provides a means of transportation for both goods and people. Option 2, Chindwin, is also a river in Myanmar, but it is not the longest. Option 3, Sittaung, and option 4, Thanlwin, are also rivers in Myanmar, but they are not the longest either. Remember that the Irrawaddy River is the correct answer because it is the longest river in Myanmar.