The attacking (electrophilic) species in sulphonation of benzene is

On reaction with acid,$4$-pyrone gives a very stable cationic product. Which of the following structures shows the protonation site in that product?

Which of the following is the strongest nucleophile?

The cyanide ion $(CN^-)$ is:

What is the decreasing order of basic strength for the following species: $\mathop O\limits^\Theta H, \mathop N\limits^\Theta H_2, HC \equiv \mathop C\limits^\Theta, CH_3 - \mathop C\limits^\Theta H_2$?

Which of the following act as nucleophiles?