Final answer:
The Nazi Party held less than 1) 10% of the Reichstag seats by 1928, as they only gained significant representation after the onset of the Great Depression.
Step-by-step explanation:
The portion of the Reichstag seats that the Nazi Party held by 1928 is not explicitly mentioned in the provided text. However, it is clear from the historical context that by 1928 the Nazi Party was still relatively insignificant in terms of parliamentary representation.
It wasn't until after the onset of the Great Depression that the Nazis gained a significant number of Reichstag seats. By the election of 1930, the Nazi Party won 18% of the seats, and by 1932, they won 37% of the national vote.
This suggests that in 1928, the Nazi representation in the Reichstag was most likely less than any of the provided percentages.
Considering the provided choices, none of them accurately describe the Nazi Party's share of Reichstag seats by 1928 since they were not a major force in the Reichstag at that time.
Therefore, if we must choose from the given options, the closest answer would be Option 1) 10%, as this would indicate the lowest level of representation, aligning with the historical information that the Nazi Party was not prominently represented in the Reichstag prior to the Great Depression.