Following the MDAS rule, where it is necessary to do multiplication or division processes first (whichever of the 2 is encounter first in the equation) before proceeding to addition and subtraction, the expression can be solved, and is done as follows:
210/7 - 18 + 4*6 - 20
Doing division and multiplication first:
30 - 18 + 24 - 20
Simplifying:
16
Therefore the answer among the choices is D.