The ornamental design for a lighting fixture arm portion, as shown and described.
FIG. 1 is a perspective view of the lighting fixture portion in accordance with the present invention.
FIG. 2 is a side view of the lighting fixture portion.
FIG. 3 is an end view of the lighting fixture portion; and,
FIG. 4 is a top view of the lighting fixture portion.
The broken lines, immediately adjacent the solid line disclosure, represent the bounds of the claim and all other broken lines are for illustrative purposes only. None of the broken lines form a part of the claimed design.