Which of the following represent the cosolvating effect?

• A

  The acidic strength HF increases in the presence of $\mathrm{B}{\mathrm{F}}_3$
• B

  The acidity of $\mathrm{N}{\mathrm{H}}_4^{+}$ is enhanced in the presence of $\mathrm{C}{\mathrm{u}}^{2+}$
• C

  The acidity of ${\mathrm{H}}_3\mathrm{B}{\mathrm{O}}_3$ is increased in the presence of glycerol
• Correct answer
  D

  All of the given are examples of cosolvating effect

Hence, the correct answer is option: 'D'.