The number of tables among the following for which HIGH_PRIORITY have an effect are . . . . . . . .

MyISAM, MEMORY, MERGE

This question is asking about how the HIGH_PRIORITY keyword affects different types of tables in MySQL. Let's break it down:

HIGH_PRIORITY is a keyword used in MySQL queries. It tells the database to prioritize the current query over others waiting in the queue. This can be helpful if you need a specific query to execute quickly, but it's important to use it with caution.

Now, let's look at the table types mentioned:

* MyISAM: This is a storage engine in MySQL. HIGH_PRIORITY does not affect MyISAM tables.
* MEMORY: This is also a storage engine in MySQL, known for its speed. HIGH_PRIORITY does have an effect on MEMORY tables.
* MERGE: This is a table type that combines multiple MyISAM tables into a single logical table. HIGH_PRIORITY does have an effect on MERGE tables.

Therefore, the answer is Option C: 2

HIGH_PRIORITY has an effect on MEMORY and MERGE tables.