A function is defined by the table below. If the pattern continues, what is f(6)?

The table shows outputs $3, 7, 11, 15$ for inputs $1, 2, 3, 4$, so each step in $x$ adds $4$ to $f(x)$. The rule is $f(x)=4x-1$, which checks: $4(1)-1=3$ and $4(4)-1=15$. Then $f(6)=4(6)-1=23$, choice H. Continuing the pattern gives $f(5)=19$ and $f(6)=23$. Choice F ($19$) stops one step short and is $f(5)$. Choice G ($21$) adds only $2$ for the last step. Choice J ($25$) adds $2$ too many (or uses $4x+1$). A constant difference means the function is linear, so the pattern is easy to extend.