The problem at hand involves a compression member that is a channel section, and we need to determine the 'outstanding length' characteristic of this section.

In the context of steel design, particularly with channel or angle sections, the term 'outstanding length' generally refers to the width of the part of the section that is not directly connected to another element or is free to buckle (project away from the connected part). In channel sections, this is significant for compression members due to potential local buckling.

The 'nominal width' of a channel section is defined as the total width of the flange of the channel. Since the flange is the part projecting out, which can be likened to an 'outstanding leg', for channel sections used as compression members, the outstanding length is, therefore, the nominal width of the section.

Let's justify this by ruling out the other options:

• Half of the nominal width: This doesn't align with standard nomenclature where the outstanding length corresponds directly to the flange width.
• From the edge to the first row of rivets: This measurement would be relevant for connected parts and not solely for determining the outstanding portion in free areas of buckling.
• None of these: Since one of the options (nominal width of the section) is actually correct, this option can also be ruled out.

Hence, the correct answer is indeed 'nominal width of the section'. the 'outstanding length' is a direct consideration of the exposed, unbraced width that primarily influences the effective buckling length in the design and analysis of such sections.