The correct answer is C: Very high speed allows digitizing high frequencies. Direct or flash conversion analog-to-digital converters are used for software defined radio (SDR) because their very high speed allows digitizing high frequencies. Flash ADCs can sample at extremely high rates, allowing direct digitization of RF signals. Flash ADCs use parallel comparators to convert all bits simultaneously, making them very fast (hundreds of MHz to GHz sampling rates). This high speed allows SDRs to directly digitize RF signals without downconversion, simplifying the receiver architecture. By digitizing at high frequencies, the SDR can process signals entirely in software. Flash ADCs are power-hungry and expensive, but their speed is essential for direct RF sampling in SDR applications.