Option 1 -> Alpha waves are associated with relaxation and flow state, not problem-solving.
Option 2 -> Beta and gamma waves characterize the 'default work' state used for problem-solving and focused tasks.
Option 3 -> Delta and theta waves are not mentioned together for problem-solving in the passage.
Option 4 -> Theta and alpha waves are associated with relaxation, creativity, and flow state, not active problem-solving.
Hence, Option 2: beta & gamma -> The passage explicitly states that "our brains are in a 'default work' state, characterized by beta and gamma waves" when "we're using our brains to solve problems and focus on tasks which require us to stay in an alert, active state" -> correct