[color=blue]You have to remember an important thing when facing multiple-choice questions.
And that is that sometimes there appear to be more than one at-least-somewhat acceptable choices. In those cases, you have to choose the best answer, which will be the only correct answer to the question.
The best answer, and therefore the only correct answer, is c.
‘a’ and ‘b’ are not used to modify embarrassed. We mostly use words like ‘a little’, ‘somewhat’ or ‘very’.
“too” can modify “embarrassed”, but it is somewhat awkward in that sentence and is not as good as “a little” and also not as good as words like ‘very’ and considerably’.