Final answer:
The shoreline varies daily as it is directly impacted by the daily tidal movements, which are influenced by the gravitational pull of celestial bodies like the moon and the sun.
Step-by-step explanation:
The factor that varies daily among the options provided is the shoreline. The shoreline is the zone that is in direct contact with the movements and changes of the tides. The high-tide line marks the farthest the water comes up on the shore and the low-tide line marks the farthest the water recedes. Everyday, due to the gravitational pull of the moon and the sun, the water levels change, which in turn affects how much of the shoreline is visible or submerged. This area is different from the coastline, which is the outer boundary of a coast, and can be seen as the furthest visible line when looking at the coast from the land. Unlike a shore, which is the land adjacent to a body of water, or a beach, which refers to the expanse of sand or pebbles along a shore, the shoreline changes throughout the day with the tides.