A hierarchical attention mechanism is the MOST appropriate technique. By first attending to relevant time steps (which might contain the subtle actions) and then attending to relevant regions within those time steps, the model can focus on the specific parts of the video that are most informative for describing the subtle actions. Increasing learning rate (A), using a larger batch size (B), adding more layers (D), and decreasing regularization strength (E) are unlikely to solve the problem of attending to subtle actions specifically. These can all improve performance, but they don't address the attention mechanism itself.