A Simplifying Assumption Can Be Made When Solving Problems in Equilibrium

A simplifying assumption can be made when solving problems in equilibrium which assumes that "x" is considerably small (approaches zero) in the sum or difference of an expression. As such, it drops out of the expression and the Algebra becomes much easier. When is it valid to use a simplifying assumption?

A) It is valid to make this assumption when the ratio: "x"/[reactant A]_{o *} 100% > 5%
B) It is valid to make this assumption when 95 out of 100 scientists chose to do so.
C) It is valid to make this assumption when "x" is comparable in magnitude to the initial concentration of the reactant A. (i.e., x ~ [A]_o)
D) It is valid to make this assumption when the ratio: "x"/[reactant A]_o < 5
E) It is valid to make this assumption when the ratio: "x"/[reactant A]_{o *} 100% < 5%