Sporadic rise in his temperature has caused us much worry.

Answer: 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