B

Step 1: Understanding the statement, a constraint relation in a problem refers to a condition or restriction that defines how some variables relate to each other.
Step 2: In most problems, especially in physics or optimization, multiple constraints can exist. For instance, in mechanics, multiple forces might act on an object leading to several constraints on its motion.
Step 3: Consider a system of equations or inequalities that must be satisfied simultaneously. The existence of more than one constraint is not only common but essential for giving a complete picture of the scenario.
Therefore, the statement claiming there can be only one constraint relation is incorrect.