Statement I states that tsunamis are small in the open ocean but can reach heights of over 30 meters when they reach a coastline. This statement is generally true. Tsunamis are indeed small and often go unnoticed in the open ocean due to their long wavelength and larger period. However, when they approach the coastline, their behavior changes due to the shallower depths, causing the wave height to increase dramatically.

Statement II states that tsunamis have long wavelengths and travel at high speeds across the open ocean. This statement is also true. Tsunamis are characterized by their long wavelengths, often several hundred kilometers. They can travel at speeds of up to 800 kilometers per hour across the open ocean.

Therefore, option 1 is the correct answer. Both statements are individually true, and statement II correctly explains statement I. As tsunamis approach the shore, their long wavelength decreases, and the wave height increases. This is because the energy of the tsunami gets concentrated in a smaller area as the water becomes shallower, leading to a buildup of the wave.