The answer is (d) Lightning.
Explanation: A tsunami is caused by large disturbances in the water, typically triggered by events like earthquakes, volcanic eruptions, or underwater landslides. Lightning, while a powerful natural phenomenon, does not create the necessary disturbances to generate a tsunami.
Why other options are incorrect:
(a) A major nuclear explosion undersea:
A large explosion underwater can displace a significant amount of water, triggering a tsunami.
(b) Earthquake:
Earthquakes, especially those occurring beneath the ocean, can cause the seabed to move, generating a tsunami wave.
(c) Volcanic eruption:
Volcanic eruptions can trigger tsunamis when large amounts of volcanic material displaces the water.