The y-intercept of the line of best fit equation \( y = -5.6x + 123 \) represents the value of \( y \) (the weight of the bar of soap) when \( x \) (the number of uses) is 0. In this case, when \( x = 0 \), the equation gives:
\[ y = -5.6(0) + 123 = 123 \]
This means that when the bar of soap is brand new (not used at all), its weight is 123 grams.
Therefore, the correct statement describing the meaning of the y-intercept is:
A. The weight of the new bar of soap was 123 grams.