A telephone has component parts from China, Thailand, South Africa and Taiwan. It is assembled in Mexico and has a $\%$ of its value assigned as "Mexican" such that it qualifies for preferential tariff rate under NAFTA. In this analysis, the rule for determining tariff applicability is:

a$.$ Tariff$-$Shift Rule.

b$.$ Domestic or Regional Value Content Rule.

c$.$ Mark Rule.

d$.$ Ultimate Purchaser Rule