Answer:
a) oxidation half-reaction.
b) oxidation half-reaction.
Step-by-step explanation:
Oxidation - Reduction Reactions => Half - Reactions.
A half reaction is either the oxidation or reduction reaction component of a redox reaction.
Remember when a compound or element is reducing, its oxidation state is 'reducing' too. This means that it is gaining electrons and that's why its charge tends to be negative; whereas when a compound or element is oxidizing, its oxidation state 'increases' meaning that it is losing electrons and that's why its charge tends to be positive.
Let's analyze each case:
a) You can see that Cl⁻ has an oxidation state of 1-, and then its oxidation state is zero because Cl2 is a neutral compound (not an ion). You can see that this oxidation state increases so it corresponds to an oxidation half-reaction.
b) Here we have to analyze Mn. You can see that Mn²⁺ has an oxidation state of 2+, and its oxidation state is 4+ because remember that the algebraic sum of oxidation states of a neutral compound must be zero and the oxidation state of O is always 2-. This oxidation state of Mn is increasing too, so it corresponds to an oxidation half-reaction.