(a) t-BuF in acidic solution > t-BuF in basic solution (t-buty fluroide is solvolysed in very acidic solution,because $F^-$ is a poor leaving group but H-bonding with a strong acid encourages its departure,this is example of electrophilic catalysis.
(b) t-BuCl > 2-chloro 2,3,3 trimethylbutane (t-Bucl solvolyse more slowly than 2-chloro 2,3,3 trimethyl butane,formation of $Me_3C^+Me-2$) alliviates steric crowding on alpha carbon.This is example of steric acceleration.
(c) t-BuCl > 2 chloro 1,1,1 trifluro 2-methyl propane $F_3C-C^+H-Me$ is destabilized by the strongly electron with drawing $F_3C$ group making the solvolysis slower than the t-BuCl.
(d) t-BuCl in 90% $H_2O$-10% dioxane t-BuCl in 90% $D_2O$-10% dioxane.-D bonds are not as stabilized as -H bonds
Hence (c) is the correct answer.