For the tert-butyl derivatives $(\mathrm{CH}{}_3){}_3\mathrm{C}-\mathrm{X}$ in a polar protic solvent, the correct decreasing order of SN1 reactivity for $\mathrm{X}$ is

Neutral water leaves far more easily than any halide (protonated alcohols react quickly), bromide is better than fluoride (weaker C-X bond and more stable anion), and hydroxide is so basic that the neutral alcohol does not ionise at all. The order is $-\mathrm{OH}{}_2{}^{+}>-\mathrm{Br}>-\mathrm{F}>-\mathrm{OH}$.