Which Modbus function code is used to READ the Holding Registers (16-bit read/write registers) of a slave?

0x03 (Read Holding Registers) targets the 16-bit read/write register space and is by far the most used code for reading analogue values such as temperatures, pressures, counters and setpoints. 0x01 is wrong because Read Coils returns 1-bit read/write outputs, not 16-bit registers. 0x02 is wrong because Read Discrete Inputs returns 1-bit read-only digital inputs. 0x04 is wrong because Read Input Registers targets a separate 16-bit READ-ONLY register table, not the Holding Registers. The full picture includes 0x05/0x06 for single writes and 0x0F/0x10 for multiple writes.

When in doubt, start with 0x03 on most slaves: it is the function code that vendor documentation maps almost everything to.