Let's begin by listing out the information given to us:
[tex]\begin{gathered} A\mleft(t\mright)=800e^{-0.86t} \\ \Delta A(t)=-0.86\cdot800e^{-0.86t} \\ \Delta A(5)=-0.86\cdot800e^{-0.86\mleft(5\mright)} \\ A(5)=-9.34 \\ A(5)=-9.3g\text{ / year} \end{gathered}[/tex]Hence, the correct option is the third option -9.3 grams per year