Answer : Option C) Oxygen in air reacts with iron content in rocks, causing them to change colour.
Explanation : Chemical weathering is a process in which the disintegration of any materials takes place due to chemical reactions mainly with water and substances dissolved in it rather than by any mechanical processes.
Here, in the given options namely;
A) Freezing water widens the cracks in rocks is a physical process where the water clogged inside the cracks of rocks gets expanded on freezing at lower temperatures. No chemical change is observed in this process.
B) Plant roots force their way into cracks in rocks and break them this is also a physical process where the plant roots search for water and nutrients into the crack of rocks and eventually breaks them. Here, no chemical changes are seen.
C) Oxygen in air reacts with iron content in rocks, causing them to change color this is completely a chemical change that is occurring; as we can see that the color of rocks are changing because of a chemical reaction of oxygen in the air and the iron content in the rocks. There is also disintegration of rock which is observed in this process. So, this can be termed as chemical weathering.
D) Strong winds peel soft rock away, leaving harder rock behind it is completely a physical process where the strong winds physically peel away the soft rock and leaves behind harder rocks. No chemical change is seen or occurs.
Hence, the correct answer is Option C) which gives complete satisfactory changes to be termed as chemical weathering.