Which instruction should be used when one needs to check if Source A is less than Source B?

The instruction to use when checking if Source A is less than Source B is "LES." This instruction specifically performs a comparison operation, evaluating whether the value in Source A is indeed less than the value in Source B. If Source A is less than Source B, the instruction typically sets a specific condition in the processor's status register, which can be used for branching or further logic decisions within a program.

The other options do not serve the purpose of comparison directly. "GRT" stands for greater than, which checks if one value exceeds another but does not help in checking for less than. "ADD" performs an arithmetic addition operation, and "MUL" facilitates multiplication; neither of these can be utilized for comparison purposes. Therefore, "LES" is the correct instruction to evaluate the specified conditional statement efficiently.