If $a_{1}, a_{2}, a_{3},.....$ are in HP and $f (k)$ = $\displaystyle\sum_{r=1}^{n}\;a_{r}-a_{k}$, then $\large\frac{a_{1}}{f (1)}$, $\large\frac{a_{2}}{f (2)}$, $\large\frac{a_{3}}{f (3)}$,...,$\large\frac{a_{n}}{f (n)}$ are in

Question

$(a)\;AP\qquad(b)\;GP\qquad(c)\;HP\qquad(d)\;None$