7974. Sporadic rise in his temperature has caused us much worry.

✅ The correct answer is option C.
Frequent : occurring or done many times at short intervals.

Irksome : irritating; annoying.

Irregular : not even or balanced in shape or arrangement.

Scattered : occurring or found at intervals or various locations rather than all together.

Sporadic : occurring at irregular intervals or only in a few places; scattered or isolated.

Synonym of Sporadic is Irregular