How do platypuses and Guiana dolphins use their electric sense to find hidden prey?

Answer: They detect weak electric fields made by the nerves and muscles of hidden prey.

They read echoes from clicks that bounce off prey and surrounding objects. — This is echolocation, the click-based sonar that dolphins use to navigate and hunt. It reads reflected sound, not electric fields. Confusing the two senses is easy because both work at close range in water, but they are separate channels.

They generate their own electric field and detect how nearby objects distort it. — This describes active electrolocation, used by some electric fish, not by platypuses or Guiana dolphins. Those fish generate a field and read how objects distort it. That is a different strategy from quietly detecting fields already present in the water.

They detect weak electric fields made by the nerves and muscles of hidden prey. ✓ — Correct. Passive electroreception means the animal does not broadcast a field; it picks up weak voltage from living bodies. Nerves and muscles make tiny electric fields in water. A hidden shrimp or fish can reveal itself electrically even when it is not signaling on purpose.