Answer:

1 ) .Reactivity series :

It is series of decreasing order of metals' reactivity. Hydrogen is place in middle of series. Above hydrogen metals are more reactive in nature below are less. Metal which is more reactive it displaces a less reactive metal from its salt solution.

2 ) .

In order to obtaining a metal from its  oxides for metals in the middle of the reactivity series then metals towards oxides of such metals can be reduced with coke (carbon) which acts as a reducing agent.

For instance :

2 Fe₂O₃ + 3 C  ⇒ 4 Fe + 3 CO₂.

In order to obtaining a metal from its  oxides for metals in the high of the reactivity series their oxides are reduced to metals by the process of electrolysis

For instance :

Electrolysis of sodium chloride :

Na⁺ + e⁻  ⇒ Na   [ At cathode ]

2 Cl⁻ ⇒ Cl₂ + 2 e⁻   [ At anode ]