What is the most appropriate machine learning approach for this scenario?
This certainly looks like a basic optimization problem. However, looking from the Machine Learning perspective, what you describe can be framed as a multilabel classification. In that case, you treat the values of A and B as your input, and try to predict the values of those five parameters, which you mentioned:

A, B -> x1, x2, x3, x4, x5

The only pitfall is that 100 records can be insufficient to train a proper classification model, but if you can obtain more training data, this approach seems solid to me.