Which of the following is a nucleophile?

Which one of the following series contains only electrophiles?

What are electrophiles and nucleophiles? Explain with examples.

Classify the following species as Nucleophilic or Electrophilic:
$NO_2^+$,$OH^{-}$,$CH_3NH_2$,$NH_3$,$Br^{+}$,$CH_3-CO^{-}-CH_3$

$(a)$ Explain the following terms and provide the difference between them:
$(i)$ Reagent species
$(ii)$ Substrate species
$(b)$ $(i)$ What is the mechanism of a reaction?
$(ii)$ What are the uses of a reaction mechanism?
$(c)$ Depict the general reaction that occurs between a substrate and a reagent in an organic reaction.

What is the correct order of nucleophilicity for the following species?