In the reaction of calcium with chlorine, which atom is oxidized?
a. calcium
b. chlorine
c. both
d. neither

The reaction between calcium and chlorine is given by the following equation ->

Ca+2 + Cl - -> CaCl2

Lets find the oxidation number of each atom.

Ca has an oxidation number of +2 as oxidation number is equal to charge.
Cl has an oxidation number of -1 since oxdiation number is equal to charge.
Lets now find the oxidation number of CaCl2. Since it's a compound the oxidation number of all atoms have to be equal to 0.
To prove that we know the oxidation number of Calcium and Chlorine.

Ca + Cl2 = 0
+2 + 2 (-2) = 0
+2 -2 = 0
0=0 therefore the oxidation number of CaCl2 is 0

Ok now lets asign this to the reaction.

Ca+2 + Cl - -> CaCl2

+2          -1            0

Check if the equation is balanced here it is not. ( In a question you are usually required to balance the equation using oxidation numbers ). It doesn't affect  which element is oxidised.  But I'll show you here any way.

Ca+2 + Cl - -> CaCl2

+2          -1            0

Calcium gains 2 electons ( e-)  where
Chlorine looses 1 electron. Therefore we need two chlorine atoms for each calcium.

Ca+2 + 2Cl- -> CaCl2

To find which atom is oxidised we need to know what oxidation means.

Oxidation in terms of electron transfer it is the loss of electrons.
and 
Reduction is the opposite ( gain of electrons )

Calcium gains two electrons therefore it is reduced.
Chlorine looses one eelectron therefore it is oxidised.

Therefore the answer is (b).

Hope this helps ;).