At transform boundaries, the primary type of seismic activity that occurs is 2. Earthquakes.
Transform boundaries are regions where two tectonic plates slide past each other horizontally. The friction between the plates can lead to the accumulation of stress, which, when released, causes earthquakes.
While landslides (1) can occur in areas affected by earthquakes, they are not specific to transform boundaries. Tsunamis (3) generally result from underwater earthquakes, especially at subduction zones, but are not directly caused by the sliding motion at transform boundaries. Volcanic eruptions (4) are also not typical at transform boundaries, as they are more commonly associated with divergent and convergent plate boundaries.
Therefore, the most accurate answer regarding seismic activity specific to transform boundaries is earthquakes.