Opposite is a common option to use when you Align two faces with opposing face normals.
Sometimes you can't tell when you have opposing face normals. Use the Untrim command (click 3D Geometry and then, in the 3D Surface Tools group, click Untrim) to see the entire topology of the surface to determine the face normal.
Two parts to Align. The Change Face is yellow. The Ref. Face is green.
Align result. The Align operation stops when it meets a surface with the face normal in the same direction as the Change Face.
Untrim the Reference Face part to determine topology and the face normal direction. The bright blue inside the cylinder signifies the face normal.
Select Opposite since the face normals of the Change Face and Reference Face oppose each other. Finish the Align operation.
Untrim) to see the entire topology of the surface to determine the face normal.
