The logical answer for a puzzle is that the letter 'W'.
A rainbow is a spectrum of light that appears in the sky when sunlight is refracted through raindrops or other drops of moisture in the Earth's atmosphere. They have inspired much popular folklore, including the belief that a leprechaun can be found with a pot of gold at the end of a rainbow.
Canada has the longest coastline than any other countries in the world. It measures 1,51,020 miles long approximately. Norway, Indonesia, Greenland and Russia being in 2, 3, 4 and 5th places.
2, 4, 9 have two similar elements intersecting each other. 6, 7, 8 have two different elements one placed inside the other. 1, 3, 5 have two different elements intersecting each other.
Gear Lapping is an operation after heat treatment. Gear lapping is the process of imparting a very fine finish and high degree of accuracy to gear teeth, by using a lapping tool and applying a fine-grained abrasive between a work material and a closely fitting surface, called a lapping plate.