The correct answer is: C. Tangent
An isoquant is a curve that shows all the combinations of inputs that produce the same level of output. An iso-cost line is a line that shows all the combinations of inputs that can be purchased with a given budget.
At an equilibrium point, the firm will be using the least-cost combination of inputs to produce the desired level of output. This means that the isoquant and the iso-cost line will be tangent at the equilibrium point.
A convex isoquant means that the firm is willing to pay more for an additional unit of input when it already has a lot of that input. A concave isoquant means that the firm is willing to pay less for an additional unit of input when it already has a lot of that input. A perpendicular isoquant is not possible, because the isoquant and the iso-cost line must intersect at some point.