The correct answer is option 4: "electrode potential is zero only at 25°C."

The standard hydrogen electrode (SHE) is a reference electrode used to measure the electrode potential of other half cells in electrochemical cells. It consists of a platinum electrode immersed in a solution of hydrogen ions at a pressure of 1 bar.

The electrode potential of the SHE is defined as zero at all temperatures. However, this is only true under standard conditions, which include a temperature of 25°C. So, at 25°C, the electrode potential of the SHE is zero.

Option 1, which states that the absolute electrode potential is not zero, is incorrect. The absolute electrode potential refers to the potential difference between an electrode and the standard hydrogen electrode, and it is always measured relative to the SHE. Therefore, the absolute electrode potential of the SHE is zero.

Option 2, which states that the absolute electrode potential is zero, is also incorrect. As explained above, the absolute electrode potential of the SHE is always measured relative to the SHE.

Option 3, which states that both absolute and standard electrode potential values are zero, is incorrect as well. The absolute electrode potential is zero, but the standard electrode potential is only zero at 25°C.

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