21.69mL - 20.70mL = .99mL Fe
7.8 g / .99 mL = 7.9g/mL
The answer is D: Saturated.
A saturated solution is one in which the exact maximum amount of solute has been dissolved. So, new solute will not dissolve in the solution. In contrast, an unsaturated solution can hold more solute, so if that option were correct, the crystal would have dissolved.
The other two terms are a bit more complicated. A supersaturated solution is one holding an amount of solute above the sustainable limit. Because of that, when more solute is added, the solution will immediately adjust, and some solute will come out of solution in a precipitate. Because the crystal isn't growing, we can eliminate this option.
A concentrated solution is one holding a relatively large amount of solute. However, you can have concentrated solutions that are saturated and unconcentrated (the word for this is dilute) solutions that aren't saturated. Therefore, we can say that because the crystal doesn't dissolve, this solution is saturated, but we can't say with certainty that it is concentrated.
Because the first three options are invalid, as described above, while the scenario does describe a saturated solution, D is the correct answer.
<h2>Input =

, water and Output =

</h2>
Explanation:
The light reactions of photosynthesis use water and produce Oxygen, NADPH.
The equation for photosynthesis :
→ 
The process of photosynthesis in two stages -
- The first stage is called the light reaction in which the light energy from the sun is captured and converted into chemical energy stored in the form of ATP and NADPH
- The second stage is the process of conversion of ATP molecules to sugar or glucose (the Calvin Cycle)
For a light reaction -
Net Input is of,
, 
Net Output is of, 
I believe the correct answer from the choices listed above is option A. The topic that the teacher is talking about would be distillation of a mixture. Gasoline is processed by distillation. Hope this answers the question. Have a nice day.