The dashed arrow on the number line starts at seven-eighths (7/8) and ends at two-eighths (2/8). To describe this movement mathematically, we can see it as a subtraction operation.
Starting from 7/8 and going to 2/8 can be expressed as:
\[ 7/8 - 5/8 = 2/8 \]
Therefore, the most appropriate option for describing the movement on the number line is:
subtraction of 5/8.