Step 1: Understand the word ``Artificial''.
Artificial refers to something that is man-made or not natural.
Step 2: Identify the opposite meaning.
The opposite of man-made is something that occurs naturally.
Step 3: Analyze the options.
Red is a color.
Truthful relates to honesty.
Solid refers to physical state.
Natural directly means not artificial.
Step 4: Conclusion.
Therefore, the exact opposite of artificial is Natural.