Explanation:

The correct answer is option 'A', i.e., "Except for."

The given sentence talks about the Cannes Film Festival, which is the biggest event that takes place in May in the South of France. The word "except for" is used to introduce an exception to the preceding statement. Here, the exception could be any other event that is bigger than the Cannes Film Festival. Thus, the sentence would make sense as follows:

"Except for the Oscar Reward, the Cannes Film Festival is the biggest event which takes place in May in the South of France."

Therefore, option 'A' is the correct answer.

Alternative Options:

Let's look at the other options and why they are incorrect:

Option 'B': "Such as" is used to give examples or to introduce a list of things. However, in this sentence, the purpose is not to give examples, so this option is incorrect.

Option 'C': "Lest" means "in case something undesirable happens." This word is used to introduce a negative consequence that should be avoided. However, there is no negative consequence in the given sentence, so this option is incorrect.

Option 'D': "Unless" means "except on the condition that." This word is used to introduce a condition that must be met for something to happen. However, there is no condition in the given sentence, so this option is incorrect.

Option 'E': "But" is used to introduce a contrast or an exception. However, the sentence already has a contrasting word "biggest," so using "but" would be redundant. Therefore, this option is incorrect.

Conclusion:

The correct answer is option 'A' because it introduces an exception to the preceding statement. The other options are incorrect because they do not fit the context of the given sentence.