When the first periodic table of the elements was published, the elements were arranged by atomic mass. Although several elements seemed out of order, later discoveries showed the order to be correct. Which event most likely helped the development of the periodic table?
Scientists changed the atomic masses to better fit the pattern.
Scientists refused to consider other ways to organize the elements.
New technology was developed that modified the masses.
New information changed the understanding of the atom’s structure.

New information changed the understanding of the atom’s structure.

Explanation:

The periodic table is the list of chemical elements that are displayed according to their atomic numbers, electronic configuration, and also their chemical properties so as to make their identification easier and much simpler to be studied. this arrangement allows for better identification and easier access to the elements.

As stated in the given passage, elements were arranged by atomic mass when the first periodic table was published. Despite the irregularity that seemed out of the ordinary for some elements, the order was kept intact. And this was so because, with the coming of new technology and the development of the process in identifying them, new information led to the change in understanding the structure of the atoms which seem to verify the correctness of the order.  

and your question what topic are you in trouble with?

isn't this axel talking to his uncle?