Alcohols react with concentrated HBr much faster than with $NaBr$ alone because

AHBr supplies a stronger nucleophile than $Br X^{-}$
Bthe acid makes the carbon atom of the C-O bond more electron rich
CHBr converts the alcohol into an alkoxide ion, which is a better nucleophile
Dprotonation of the $- OH$ group gives $- OH X_{2} X^{+}$ , so the leaving group becomes neutral water instead of $OH X^{-}$

Answer: D

OH X^{-}

is a poor leaving group (conjugate acid pKa about 15.7). Protonation by HBr converts it into

H X_{2} O

(conjugate acid

H X_{3} O X^{+}

, pKa about -1.7), a neutral and much better leaving group. Bromide then displaces water.