A relationship is a function only if there is no more than one $y$-value for each $x$-value. Check the $x$-values in both tables to see to find any that appear multiple times. When you find a value for $x$ that appears multiple times, check the corresponding $y$-values. If the $y$-values are different, then the relationship is not a function.

Relationship (b) is not a function. When $x$ equals $2$, $y$ can equal $0$ or $1$.