The myisamchk variable that represents the size of buffer used to hold index blocks is . . . . . . . .

Answer: Option A

This question asks about a MySQL variable that controls the size of the buffer used for storing index blocks. Index blocks are used to speed up data retrieval in your database.
Let's break down the options:
Option A: key_buffer_size: This variable specifies the size of the buffer used to cache index blocks for MyISAM tables. This is the correct answer!
Option B: read_buffer_size: This variable controls the size of the buffer used for reading data from the disk when fetching rows. It's not directly related to index blocks.
Option C: sort_buffer_size: This variable defines the size of the buffer used for sorting data during operations like sorting, creating indexes, or joining tables.
Option D: write_buffer_size: This variable sets the size of the buffer used for storing data before it's written to the disk. It's not directly related to index blocks.
Therefore, the answer is Option A: key_buffer_size.