Which animal makes the lowest frequency?

Answer 1

Quite obviously the sea cucumber, it generates .0578 decibles every time it does its mating call

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That is neither "obvious" nor sensible. The decibel scale is used for sound pressure-levels, not frequency (that's counted in Hertz); you would have to state the dB reference level used as your are quoting the marine not human-hearing world; and quoting a sound-level to 4 decimal places is meaningless anyway.

In short - don't try to invent answers to questions you don't understand.

And no, I don't know the actual answer, but whales and some large land animals including elephants use infrasound (<20Hz) to communicate with each other.

Incidentally I do not know if the sea-cucumber, an invertebrate, can make any sound at all - many invertebrates, including aquatic ones, use scent or an underwater equivalent when they are feeling frisky. If it did it would stay rather lonely because a sound at <0.1dB re even the 20micro-Pa that is the human ear's threshold of hearing, would be a mere whisper lost barely a metre or so at best from the poor creature! Marine work uses 1micro-Pascal as its dB scale reference: 26dB lower still.