An oxidizing agent is a substance that can:
A.Gain electrons
B.Lose an electronegative radical
C.Undergo a decrease in the oxidation number of one of its atoms
D.Undergo any one of the above changes

Hint: An oxidizing agent, or an oxidant, is an agent which gets reduced in a chemical reaction. Electron acceptors participate in electron-transfer reactions. In this context, the oxidizing agent is called an electron acceptor and the reducing agent is known as an electron donor.

Complete step by step answer:
-An oxidizing agent (oxidant, oxidizer) is a substance which can oxidize other substances  or accept their electrons.
-An oxidizing agent is a chemical species which undergoes a chemical reaction in which it gains one or more electrons. In that sense, it is one component in an oxidation-reduction or (redox) reaction. We can even say that an oxidizing agent is a chemical species which transfers electronegative atoms, usually oxygen, to a substrate. Combustion, many explosives, and organic redox reactions may involve atom-transfer reactions.
-The term oxidation is used for the gain of oxygen and the term reduction is used for the loss of oxygen.
Now, we understand this with the help of an example:
$F{e_2}{O_3} + 3CO \to 2Fe + 3C{O_2}$
-So, in this example since $Fe$oxidizes, carbon monoxide to carbon dioxide and itself gets reduced from $F{e^{ + 3}} \to F{e^{ + 2}}$ oxidation state on its own. Thus iron (iii) is the oxidizing agent of this reaction.
-Thus, the compound that undergoes oxidation itself to reduce some other compound is a reducing agent and the compound that undergoes reduction to oxidize another compound is called an oxidizing agent.
An oxidizing agent is a substance that usually:
-Will gain electrons because it undergoes reduction itself.
-Loses an oxygen atom or electronegative atom.
-Gains a hydrogen atom or electropositive atom.
-Loses an increase in its oxidation number.
So, correct answer is (D).

Note:
-Consider the reaction,
$2Mg + {O_2} \to 2MgO$
-In this reaction, the oxygen molecule is making magnesium to lose electrons, and hence oxygen acts as an oxidizing agent in the reaction.
-Examples of some of the common oxidizing agents are oxygen, ozone, hydrogen peroxide, sulphuric acid, nitric acid, perchloric acid, potassium chlorate, acidified potassium dichromate, etc