Final answer:
The composition of the resin cement has the greatest effect on bond strength because it determines the strength of the chemical bonds formed.
Step-by-step explanation:
The variable that has the greatest effect on bond strength is the composition of the resin cement. The strength of the bond between two substances is determined by the strength of the chemical bonds between them. The composition of the resin cement directly affects the type and strength of the chemical bonds it forms with the other materials involved in bonding.
For example, a resin cement with a strong composition that forms strong chemical bonds with the bracket and tooth surface will result in a stronger bond. On the other hand, a resin cement with a weak composition will result in a weaker bond. Therefore, the composition of the resin cement is the most important variable in determining bond strength.