Answer: C) angle 2 and angle 7
Imagine the parallel lines form the boundaries of river. Angles 3, 4, 5 and 6 are considered interior angles as they are in the river. Angles 1, 2, 7, 8, 9 and 10 are exterior because they're outside the river.
Angles 2 and 7 are on opposite sides of the transversal cut that I've highlighted in red (see diagram below). So that's why they're considered alternate. Combine "exterior" and "alternate" to get "alternate exterior"