The audiences for crosswords and sudoku, understandably, overlap greatly, but there are differences, too. A crossword attracts a more literary person, while sudoku appeals to a keenly logical mind. Some crossword enthusiasts turn up their noses at sudoku because they feel it lacks depth. A good crossword requires vocabulary, knowledge, mental flexibility and sometimes even a sense of humor to complete. It touches numerous areas of life and provides an "Aha!" or two along the way._______________________________
Option (b) gets eliminated because of the idea of ‘popularity’ in it. This is a new idea and will require some reader intervention to support it. Reader intervention is not required in the last sentence of a paragraph.
Option (c) gets eliminated because of ‘even by children’- we need to assume that children lack ‘vocabulary etc. etc. mentioned in the paragraph.
Option (d) contradicts the paragraph. The paragraph says it appeals to a logical mind.
Option (a) effortlessly closes the paragraph. The comparison between Crossword and Sudoku is completed and the purpose of the paragraph is fulfilled.
Hence, the correct answer is option (a).