Put the following metric units in order from smallest to largest: pl, ml, dl, kl.

Explanation:

These are multiples and submultiples of the unit liters.

                          pL, mL, dL and kL

                          pL<mL< dL< kL

Units can be expressed in form of multiples and submultiples form using the following prefixes:

  Note: submultiples are lower than multiples:

            submultiples              10⁻¹            deci(d)

                                               10⁻³            milli(m)

                                               10⁻¹²            pico(p)

        multiples           10                  deca(da)

                                  10³                  kilo(k)

the answer would be a