Mike is ill.His mother feels__(worry)

worried

feel系动词后面应届形容词 所以应用worry的形容词形式worried 意味担心的 焦虑的

worryvt. 使烦恼,使焦虑
His mother 是worry的受动者,故用worried.
Mike is ill. His mother feels worried.

worried.　形容词，焦虑的。