The correct answer is: A. FHSS and DSSS.
Frequency-hopping spread spectrum (FHSS) and direct-sequence spread spectrum (DSSS) are two spread spectrum techniques that were used in the original IEEE 802.11 standard.
FHSS works by rapidly changing the frequency of the transmitted signal over a wide range of frequencies. This makes it difficult for eavesdroppers to intercept the signal, as they would need to know the hopping pattern in order to decode the signal.
DSSS works by spreading the transmitted signal over a wide range of frequencies by multiplying it with a pseudorandom noise (PRN) sequence. This makes it difficult for eavesdroppers to intercept the signal, as they would need to know the PRN sequence in order to decode the signal.
Both FHSS and DSSS have their own advantages and disadvantages. FHSS is more resistant to narrowband interference, but it is more susceptible to multipath fading. DSSS is less resistant to narrowband interference, but it is more resistant to multipath fading.
The original IEEE 802.11 standard used both FHSS and DSSS in order to provide a balance of performance and robustness. However, the 802.11b and 802.11g standards only use DSSS, as this technique has been shown to be more efficient in terms of bandwidth usage.