Fill in the blanks with the most appropriate option:-

My sister_____ annoying today, but usually she is nice.

is being

The most appropriate option to fill in the blanks is is being.

The sentence "My sister is being annoying today, but usually she is nice" is in the present continuous tense, which is used to talk about an action that is happening now and is ongoing. The other options are not as appropriate.

• Has is the present perfect tense of the verb "to be". This does not make sense in the context of the sentence, as the speaker is talking about an action that is happening now, not an action that happened in the past.
• Is is the present tense of the verb "to be". This does not make sense in the context of the sentence, as the speaker is talking about an action that is happening now, not a state of being.
• Was is the past tense of the verb "to be". This does not make sense in the context of the sentence, as the speaker is talking about an action that is happening now, not an action that happened in the past.