The correct answer is C: Using a more efficient digital code. Data rate can be increased without increasing bandwidth by using a more efficient digital code that encodes more bits per symbol. More efficient codes pack more information into each symbol. For example, if you're using 2-FSK (2 bits per symbol), you could switch to 16-QAM (4 bits per symbol) to double the data rate without increasing bandwidth. More efficient codes use more signal states (amplitude/phase combinations) to represent more bits per symbol. This is how modern digital systems achieve high data rates in limited bandwidth. Forward error correction can improve reliability but doesn't increase data rate. Increasing ADC resolution doesn't increase data rate either.