To reactivate indexes the command used is . . . . . . . .

This question is asking about how to reactivate indexes in MySQL. Indexes are like a table of contents for your data, helping MySQL find information quickly. Sometimes you might need to disable indexes to speed up data modifications, but then you'll need to reactivate them afterwards.

Let's look at the options:
Option A: DISABLE KEYS - This command is used to disable indexes, not reactivate them.
Option B: DISABLE INDEXES - This command is also used to disable indexes.
Option C: ENABLE KEYS - This is the correct answer! It's used to reactivate indexes that were previously disabled.
Option D: ENABLE INDEXES - This command doesn't exist in MySQL.

So, the answer is Option C: ENABLE KEYS.