Which of the following is not a strong acid?

What is the $pH$ value of a basic aqueous solution?

When soap is scrubbed on a stain of curry on a white cloth,why does it become reddish-brown,and why does it turn yellow again when the cloth is washed with plenty of water?

Which of the following gives the correct increasing order of acidic strength?

The metal salt $A$ is blue in colour. When salt $A$ is heated strongly over a burner,a substance $B$ present in it is eliminated and a white powder $C$ is left behind. When a few drops of a liquid $D$ are added to powder $C$,it becomes blue again.
$(a)$ Identify $A, B, C, D$.
$(b)$ Write the chemical equations involved.
$(c)$ Give an example of a salt which also shows the above property.

Dry hydrogen chloride gas does not turn blue litmus red whereas hydrochloric acid does. Give one reason.