Answer
(C) $f(n)=27-3n$
Work Step by Step
From the graph, we have
First term of the sequence $a_{1}=24$ and
Common difference $d=21-24=-3$.
$n$th term is given by the function
$f(n)=a_{1}+(n-1)d$
$\implies f(n)=24+(n-1)(-3)$
$\implies f(n)=24-3n+3$
$\implies f(n)=27-3n$.
The correct one is option C.