Which of the tables represents a function?

table a
input output
7 2
8 1
5 6
table b
input output
5 2
3 5
5 6
table c
input output
1 2
1 9
7 3
table d
input output
2 6
6 7
6 2

table a
table b
table c
table d

Step-by-step explanation:

A function is a relation between two sets, which are called domain, expressed with x-variable and range, expressed y-variable.

The obligatory characteristic of a function is that every element in the domain only can have one element related in the range, that is, if we enter x = 5 in a function, we will just obtain one value back, like y = 6, in this context we cannot obtain a different value for y, if that happens it means that the relation is not a function.

So, in this case, Table A fulfil this definition.

Table B violate this functions definition, because x = 5 has to images in the range y = 2 and y = 6, both are image of a single domain value, therefore, it's not a function.

Table C and Table D do the same thing, for one domain value there's two range values.

Therefore, only Table A represents a function.