Statement I: In the context of effect of water on rocks, chalk is known as a very permeable rock whereas clay is known as quite an impermeable or least permeable rock. Statement II: Chalk is porous and hence can absorb water. Statement III: Clay is not at all porous. Which one of the following is correct?

Answer: (c) Only one of II and III is correct and that explains I

Chalk is porous and its pores connect, so water passes through (II ✓, explains I). Clay’s pores are tiny and don’t let water flow (III ✗).

The idea

Porosity measures space available to hold water; permeability measures how easily water moves through it. Movement needs connected openings of sufficient size. A material can therefore hold plenty of water while letting it pass very slowly.

Water enters pore spaces → Pore connections provide possible paths → Narrow pore throats resist movement → Fractures can provide wider paths → Rock structure determines how readily water flows

Why the others are wrong

(a) Statement III is wrong — clay is highly porous.

(d) Rejects II too. Chalk really is porous and its pores connect — that is exactly why it is permeable.

The trap

Statement III uses the absolute “not at all porous”. People mix up porosity and permeability; clay has plenty of pores.

Elimination technique

Clay has pore spaces, so Statement III is false and (a) and (b) fall. Chalk can absorb water, so Statement II is true and (d) also falls. This uses the physical difference between storage and flow, rather than the wording alone.

Also know

An aquifer stores and transmits usable groundwater; an aquitard transmits it slowly. Sand and gravel often make good aquifers, while clay-rich layers restrict flow. Chalk is a fine-grained limestone: its pores store water, but fractures can be especially important for groundwater movement. Granite can transmit water through joints even when its unbroken rock has little pore space.