Rapid cooling of molten rock at the surface creates a glassy texture, because crystals don’t have time to form. Obsidian is the natural volcanic glass that forms when lava erupts and cools rapidly at the surface or just beneath it. That quick cooling gives obsidian its smooth, glassy appearance and conchoidal fracture. The other options don’t fit this scenario: limestone forms from compacted shells and minerals in sediment; slate forms from metamorphism of shale under directed pressure; granite forms underground and cools slowly to develop visible mineral grains. So obsidian is the example of igneous rock formed by rapid surface cooling.

Erosion by streams is the process where flowing water removes and transports sediment. As water moves, it has enough energy to pick up particles from the bed and banks, carrying them downstream either in suspension, along the bottom, or by bouncing and rolling (saltation). This continuous removal and relocation of material shapes river valleys and other landscapes. Lithification is the transformation of loose sediment into rock, not its transport. Lahars are volcanic mudflows, a specific hazardous flow with water and volcanic debris. Subduction is a tectonic plate process, not sediment transport by water. So the term that best fits “the removal and transport of sediment by flowing water” is erosion by streams.

S waves rely on shear motion, which requires a material that can resist shear stress. Liquids have virtually zero shear stiffness, so they cannot support transverse (shear) waves. P waves, by contrast, are compressional and move through any medium that can transmit pressure changes (solids, liquids, and gases). Surface waves also depend on solid boundaries and rigidity, but the fundamental reason this wave won’t pass through a liquid is the lack of shear strength. So the wave that cannot travel through liquids is the one that moves by shear.

Tides arise from gravitational forces that external bodies exert on Earth's oceans, and the effect comes from the way gravity changes across the planet. The Moon and the Sun both tug on the oceans, creating bulges on the side of Earth facing them and on the opposite side as the pull weakens with distance. As Earth rotates, these bulges move, producing the regular rise and fall of sea level. The Moon has the stronger influence because it is much closer to Earth, so the gravitational gradient it creates is larger. The Sun, while far more massive, is so far away that its tidal effect is smaller, though still noticeable. When the Sun and Moon align, their tides reinforce each other, making higher high tides and lower low tides. When they’re at right angles, the tidal effects partially cancel, producing smaller tides. So the cause is the gravitational forces of the Moon and Sun, with the Moon contributing the largest portion of the tidal effect.