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I believe the answer is the third option. Hope this helps! Please tell me if I am wrong or if there was an error in my answer... also sorry this answer is late.
Rock is a combination of two or more minerals, and it does not possess any specific chemical composition. It is present naturally in the environment. Some examples of rocks are limestone, granite, etc.,Limestone may contain minerals like calcite and aragonite. Each mineral has a specific chemical composition with a well defined crystalline structure.
Therefore, the correct option would be a. rock as rock is the naturally occurring solid mass made up of different types of minerals.
I searched it up on Google and I got 473.176 Hope this helps.
Answer:
The concentration is 
Explanation:
From the question we are told that
The absorbance is 
The length is 
Generally absorbance is mathematically represented as

where
is the molar absorptivity of FeSCN2 with a value 
and
is the equilibrium concentration of FeSCN2
So

substituting values

