As per this theory, once upon a time, our earth was once a single, giant landmass, named as called Pangaea. Identify this theory.

Continental drift

The correct answer is Option (3) - Continental drift

In 1912, German scientist Alfred Wegener proposed a theory he called continental drift. According to Wegener's theory, Earth's continents once formed a single, giant landmass, which he called Pangaea. Over millions of years, Pangaea slowly broke apart, eventually forming the continents as they are today.