Answer:
3. Hopeful, because our understanding of the atom is advancing.
Step-by-step explanation:
The attitude of a writer refers to the tone that is used by the writer in narrating or writing the text. This means that specific words are used by writers to reveal their tone, their attitude, the feelings towards the topic.
In the given passage, the author delves into how our understanding of atoms has developed from ancient times to what we now know. And in stating that "thanks to these scientists and others, our understanding of the atom is more complex than it ever was, and it continues to evolve and change." This line shows that the writer's tone is hopeful. The use of words such as "evolve, change" reveals the hopeful tone of the writer.
Thus, the correct answer is option 3.