The default value in seconds in the system variable 'long_query_time' is . . . . . . . .

This question asks about the 'long_query_time' system variable in MySQL. This variable is used to determine which queries are considered "long" and should be logged.
The 'long_query_time' variable is measured in seconds.
The question wants to know the default value set for this variable.
Think about it this way: If a query takes longer than this default time, it's flagged as a long query and recorded in the slow query log.
Now, let's look at the options:
Option A: 5
Option B: 10
Option C: 20
Option D: 60
The correct answer is Option D: 60.
By default, a query is considered long if it takes more than 60 seconds to complete.