Pumice is porous and formed from lava.

Explanation:
- Pumice is a type of volcanic rock that is formed from lava.
- It is created when lava with a high gas content erupts from a volcano and cools rapidly.
- The rapid cooling of the lava traps gas bubbles within the rock, giving pumice its characteristic porous and lightweight nature.
- Pumice is typically light-colored, ranging from white to gray, and has a rough, abrasive texture.
- It is commonly used as a lightweight aggregate in construction materials, as an abrasive in cleaning and polishing products, and even as a natural exfoliant in beauty products.
- Pumice is highly porous, which means it has many open spaces or holes within its structure.
- These pores allow pumice to float on water, making it useful in applications such as buoyancy aids and filtration systems.
- The porosity of pumice also allows it to absorb and retain liquids, making it effective in gardening and horticulture as a soil amendment or water-retaining additive.
- In contrast to pumice, other options mentioned in the question are not formed from lava or do not possess the same porous characteristics.
- Sandstone is a sedimentary rock composed of sand-sized grains held together by a natural cementing material.
- Granite is an igneous rock that is primarily composed of quartz, feldspar, and mica minerals.
- Obsidian is a naturally occurring volcanic glass that forms when lava cools rapidly.
- Therefore, the correct answer to the question is option B, pumice.

Pumice