The best reagent to convert pent-$3$ -en-$2$ -ol into pent-$3$ -en-$2$ -one is:

The goal is to oxidize a secondary alcohol to a ketone without attacking the double bond.PCC or ${\text{CrO}}_3$ in glacial acetic acid are mild oxidizing agents that selectively oxidize alcohols without affecting $\text{C}=\text{C}$ bonds.Acidic ${\text{KMnO}}_4$ or ${\text{K}}_2{\text{Cr}}_2{\text{O}}_7$ would likely cleave or oxidize the double bond.