A return on assets of $5\mathrm{.}15\%$ means that a company is earning:

a$.$ a $$5\mathrm{.}15$ return on every $$100$ of assets minus liabilities.

b$.$ a $$5\mathrm{.}15$ return on every $$100$ of total assets.

c$.$ a $$5\mathrm{.}15$ return on every $$100$ of current assets.

d$.$ a $$5\mathrm{.}15$ return on every $$100$ invested in long$-$term assets.