Question: Which shell query displays all citizens with an age greater than or equal to 21?

1. `db.citizens.select('WHERE age >= 21')`
2. `db.citizens.where('age >= 21')`
3. `db.citizens.find('WHERE age >= 21')`
4. `db.citizens.find({age: {$gte: 21}})`

Answer: The correct answer of the above question is Option D:`db.citizens.find({age: {$gte: 21}})`