Sporadic rise in his temperature has caused us much worry.

The correct answer is

Sporadic means happening irregularly or at intervals. In the context of the question, the sporadic rise in his temperature means that the temperature is not rising at a regular rate. This can be a cause for concern, as it could be a sign of an underlying medical condition.

The other options are not correct. Frequent means happening often or regularly. Irksome means annoying or tiresome. Scattered means spread out or dispersed.