Answer:
D. but
Explanation:
Option D is the correct answer.
"But" is the word that best replaces "therefore" in the sentence. The use of "but" is seen when it is used to introduce another statement that is added which is different from what was earlier said.
From the sentence, it is seen that adding "but" joins the two independent clauses together. The first part of the sentence talks about early photography being black and white. While the second part introduces when the color images arrived.
