What would be the opposite of "subside"?

The opposite of "subside" is best represented by the notion of "to escalate." When something subsides, it means that it is decreasing in intensity or severity, such as a storm calming down or a fever reducing. In contrast, to escalate implies an increase or intensification of something. For instance, if tensions or conflicts are escalating, they are becoming more intense, rather than decreasing.

The other options suggest a reduction in intensity but do not encapsulate the idea of a rise or increase that "escalate" conveys. "To relax" and "to calm" both indicate a soothing or lessening of tension but do not represent an increase, while "to diminish" similarly suggests a decrease rather than a rise in any context. Thus, “to escalate” appropriately captures the opposite action of “to subside.”