Select the most appropriate option to fill in the blank.

Due to shortage of time, I was unable ______ the task satisfactorily.

to complete

The most appropriate option to fill in the blank is to complete.

The verb "unable" is followed by the preposition "to" when it expresses the inability to do something. In this sentence, the speaker is unable to complete the task satisfactorily because of a shortage of time.

The other options are not grammatically correct.

• "In completing" is not a grammatically correct phrase. The preposition "in" should be used to indicate a location or a state of being. In this sentence, the speaker is not in the process of completing the task.
• "For completing" is also not a grammatically correct phrase. The preposition "for" is used to indicate a purpose or a reason. In this sentence, the speaker's inability to complete the task is not the purpose or the reason for the sentence.
• "From complete" is not a grammatically correct phrase. The preposition "from" is used to indicate a source or a starting point. In this sentence, the speaker is not starting from the point of completing the task.