The low, ominous sound is most likely a pedal point, a technique common in baroque music that creates tension and anticipation through a sustained low pitch. Option a is the right choice.
Breakdown of options:
a. Pedal point (baroque era): This is a strong possibility. Pedal points have a long history in music, creating a sense of tension and anticipation by holding a low pitch while other harmonies change above. This technique was certainly well-established in the baroque era.
b. Pedal point (classical era): Pedal points were still heavily used in the classical era, so this option is valid but doesn't differentiate itself from option a.
c. Walking bass (romantic era): Walking bass lines, with their characteristic stepwise movement, can add a sense of unease and darkness, but they typically aren't associated with the extremely low frequencies that create a truly ominous feeling. They also originated in the romantic era, making them less likely for music at the beginning.
d. Walking bass (classical era): Similar to option c, walking bass lines in the classical era weren't typically used for ominous effects and are less likely for the initial sound.
Based on the description of a low, ominous sound, the most likely culprit is a. a pedal point (held or repeating pitch), which entered music history in the baroque era. The low pitch and sustained nature of a pedal point perfectly fit the scenario.
hence, option a is the right choice.