Improperly digested nutrients may indicate a problem of which tissue layer?

The answer to this question would be mucosa.

In mucosa layer, there is epithelial cell with hair like projection called cilia. This cell will secrete many enzymes that use to digest nutrient into a smaller molecule that can be absorbed. The cell cilia make it surface area bigger, thus can absorb nutrient faster.

A fraction equals zero if and only if its numerator equals zero. So, you have

In turn, a product is zero if either of the factors is zero:

And we have

Those values are compatible with the existence of the fraction (i.e. they don't nullify the denominator), so we can accept them as zeroes.