Answer
This is a redox reaction.
Oxidizing agent: $Ti$
Reducing agent: $Mg$
Work Step by Step
Before the reaction, $Ti$ was in $TiCl_4$.
Each chlorine has "-1" as an oxidation number, so, $Ti$ has +4.
After the reaction $Ti$ is represented as $Ti(s)$
So, it has 0 as its oxidation number.
$Ti$ reduced. (+4 --> 0), making it the oxidizing agent.
Before the reaction $Mg$ was in $Mg(l)$
Therefore, it had an oxidation number of 0.
After the reaction, $Mg$ is in $MgCl_2$
Each chlorine has "-1" as its oxidation number, so, $Mg$ has $+2$.
$Mg$ is oxidized (0 --> +2), making it the reducing agent.
