To determine whether query cache is supported by the server the variable is . . . . . . . .

Answer: Option A

This question is asking you about how to check if your MySQL server has a feature called query cache.
The query cache is a way to store the results of frequently used queries so that MySQL doesn't have to run them again and again, saving time and resources.
To find out if your server supports this feature, you need to look at a special variable.
Out of the options given, only "have_query_cache" is the correct variable that tells you if the query cache is enabled.
So the answer is Option A: have_query_cache.
The other options are not valid variables in MySQL.