To solve this question, the user needs to know how to manipulate equations with variables and fractions.

We are given the equation:

$$(a-b)/3.5 = 4/7$$

We can simplify this equation by cross-multiplying and solving for a-b:

$$(a-b) = (4/7)*3.5$$

$$(a-b) = 2$$

Now, we have the equation a-b = 2, which means that a is two more than b. Therefore, b < a.

Option B is correct because b is always less than a in this equation.

Option A is incorrect because b is not equal to a.

Option C is incorrect because b is always less than a in this equation.

Option D is incorrect because b is always less than a in this equation.

The Answer is: B. b < a.