This is a modifier + parallel construction question:

"Developed by Pennsylvania's Palatine Germans about 1750" is a modifier to what follows immediately. The question you must ask yourself is: what was developed by Palatine Germans? It was the Conestoga wagons. Therefore the correct choice must start with "Conestoga wagons". That eliminates Choice A and B.

The author then goes on say that these wagons had high wheels and a curved floor. As Madukumar_v indicated, both of these were feature of the Conestoga wagons (that achieved some objectives). The sentence must follow a parallel construction listing both of the features.

Choice C: "with high wheels..., and had a floor..." is not a parallel construction. Moreover, the sentence is fragmented: "Conestoga wagons,..., and had a floor...to prevent cargo from shifting...". This choice is therefore incorrect.

Choice D: "Conestoga wagons had high wheels..., and a floor ... to prevent cargo from shifting..." is no only parallel but also a complete sentence. It's a good Choice.

Choice E: " Conestoga wagons had high wheels...and floors curving upward..." is parallel, but awkward. The phrase "so that it prevented" makes it less desirable stylistically. Between Choice D and E, D is better.

When dealing with long sentences, it is better to simplify the sentence to understand the structure first. That helps you identify potential errors. In this sentence, simplification would allow us to see that there must be a parallel construction between phrases describing "high wheels" and " a floor that was curved". The modifier error should be obvious at the very outset.

Hope that helps.