Answer:
The correct answer is letter "C": in color.
Step-by-step explanation:
In order to make a sentence more concise, it is better to avoid redundancies, it is setting aside words that have a similar meaning within the same segment. Redundancies are typically useful for literature when the author wants to emphasize the meaning of something. Though, when talking about scientific facts, it is better not to use them because they tend to confuse the meaning of the message delivered. In:
When the scientists added the final chemical into test tube, the malodorous liquid turned green in color.
"...in color" does not add meaning to the message. If something turns green it is understood that the color has changed. Thus, to make more concise the sentence we should delete it.