Short Answer 
Terrestrial planets have solid surfaces and experience tectonic activities like earthquakes and volcanic eruptions, while gas giants lack solid surfaces and are geologically inactive, resulting in no tectonic movements.
Step 1: Understand Terrestrial and Gas Giants
Terrestrial planets possess a solid surface, allowing for physical structures such as mountains and valleys. In contrast, gas giants like Saturn, Neptune, and Uranus primarily consist of gases without a defined solid surface. Hence, they lack the geological features that characterize terrestrial planets.
Step 2: Tectonic Activities on Terrestrial Planets
Tectonic activities arise from the movement of molten material within the interiors of terrestrial planets. This internal activity leads to phenomena such as earthquakes and volcanic eruptions. Only terrestrial planets, including Earth, experience these dynamic geological processes due to their solid crusts that can shift and reshape.
Step 3: Absence of Tectonic Activity in Gas Giants
Gas giants do not exhibit tectonic activities since they lack a solid surface that can move or break apart. Their composition primarily consists of gases, resulting in a stable, unchanging exterior. Thus, without volcanic activity or shifting plates, gas giants remain geologically inactive compared to their terrestrial counterparts.