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The correct method for calculating the length of DNA is by using the total number of base pairs and multiplying it by 0.34 nanometers (nm), which reflects the helical structure of DNA. Each base pair in the double helix contributes approximately 0.34 nm to the overall length of the DNA molecule. Therefore, if you know the total number of base pairs, you simply multiply that number by 0.34 nm to get the length of the DNA in nanometers.

For example, if a DNA molecule has 3 billion base pairs, the calculation would be:

Length = 3,000,000,000 base pairs × 0.34 × 10^-9 meters/base pair,

resulting in a total length of around 1 meter when converted to appropriate units.

This method accurately accounts for the physical properties of DNA structure, specifically its dimensions in nanometers. The other choices do not properly reflect the structure or measurement units relevant for determining DNA length in relation to its base pairs, thus making them inaccurate for this calculation.