I would advise you to swap B and C columns for the reason that I will explain. Then in D2 type: =VLOOKUP(A2, B2:C4, 2, FALSE)
Finally, copy the formula for the remaining cells.
Explanation: VLOOKUP will first find the value of A2 in the range B2 to C4 (second argument). NOTE: VLOOKUP always searches the first column in this range. This is the reason why you have to swap the two columns before doing anything.
Once the exact match is found, it will return the value in the adjacent cell (third argument).
This means that, if you put 1 as the third argument, the function will return the value in the first column of the range (which will be the same value you were looking for). If you put 2, it will return the value from the second column in the range (the value in the adjacent cell-RIGHT SIDE of the found value).
FALSE indicates that you are finding the exact match. If you put TRUE, you will be searching for the approximate match.