The two options are "usually" as the primary better answer and "seriously" as the secondary possible answer.
The usage in the question is clearly as an adverb enhancing the descriptor "was good". Consider "She was normally good at covering". It is not an adverb modifying the state of being "was" as in how she was at that time. Consider "She was serious while she was good at covering".
Also, considering the chosen conjunction "but" (indicating contrary to her normally good ability to cover), the primary best answer "normally" makes the most sense. Even if the conjunction used in the question was "yet" (which might better indicate in spite of her seriousness while being good at covering), the secondary possible answer "seriously" still sounds awkward with "good" .
The secondary possible answer "seriously" can also be interpreted as the newer, very informal meaning (seriously = not joking/no kidding = very?). Technically it works, but does not fit the context of the question at all. The sentence used within the question gives no indication of the appropriateness of such an informal answer.
The better answer among the choices offered is "normally". The best answer (not) among the choices offered would be "usually" which has close meaning and makes a good comparison of support.