The enthalpies of all elements in their standard states are:(i) Unity   (ii) Zero(iii) < 0     (iv) Different for every element

The enthalpy of formation of elements in their standard states is defined as zero. This is a fundamental concept in thermodynamics, specifically in the context of standard conditions.

1. Understanding Enthalpy: Enthalpy is a measure of total heat content in a system and is typically used in the context of chemical reactions and processes.

2. Standard State Definition: The standard state of an element refers to its most stable form at a pressure of 1 bar and a specified temperature (usually 25°C or 298 K). For example, the standard state of oxygen is O₂ gas and that of carbon is graphite.

3. Implications of Standard Enthalpy: By convention, the enthalpy of formation of any element in its standard state is set to zero. This serves as a reference point for calculating the enthalpy changes in reactions involving that element.

Thus, the correct answer to the question is (ii) Zero. This notation facilitates the calculation of the change in enthalpy for reactions forming compounds from elements, allowing chemists to easily reference enthalpy values in thermodynamic calculations without needing to account for the enthalpy of elements themselves.