Which relation is a function?

a. {(1, 2); (1, 3); (1, 4); (1, 5)}
b. {(1, 2); (2, 3); (3, 4); (4, 5)}
c. {(1, 2); (2, 3); (3, 4); (2, 5)}
d. {(1, 2); (3, 2); (3, 3); (4, 2)}

b?

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The best and most correct answer among the choices provided by the question is B. {(1, 2); (2, 3); (3, 4); (4, 5)} .    
      
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Hello there.

Which relation is a function?

B. {(1, 2); (2, 3); (3, 4); (4, 5)}