A is the correct answer.
To be precise, the pronoun here is emphatic and, therefore, optional- unless you want to make it clear that you will do the work without help. In other words: "He thinks I�ll do that work." is grammatically correct too. However, it doesn�t imply that you�ll do the work on your own.
Reflexive pronouns, on the other hand, aren�t optional and don�t mean "without help". Examples:" I cut myself". "She hurt herself". The damage could have been the result of an accident they�re not responsible for.
Remember that reflexive and emphatic pronouns have to concide with the performer of the action the pronoun is referring back to.
Examples:
The dog dug the hole itself. (Emphatic. I can also say "The dog dug the hole".) > Itself refers to the dog not the hole.
We finished the project ourselves. (Emphatic- I can also say "We finished the project.") > ourselves refers to "We".
Susan killed herself. ( Reflexive, not optional. She didn�t kill others but her own person.)
Peter saw her look at herself in the mirror. ( Reflexive, not optional. Herself refers to the woman who saw her reflection in the mirror not to Peter.)
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