Dr. Gomez will not know which patients actually received the drug.
For this problem, you simply need to know the definition of a double blind study. In a double blind study, neither the participants or the experimenters know who is getting the placebo, and who is getting the medication. This helps prevent inadvertent bias on the part of the experimenters. So let's look at the available choices and see what matches the definition.
the patients will know if they are receiving the drug or the placebo: Obviously wrong. None of the experimental protocols allow for the patient to know if they're receiving the drug of a placebo.
only dr. gomez will know which patients actually received the drug: This would be correct if it were a "single blind study", but since this is a "double blind study", this is the incorrect answer.
dr. gomez will not be told the operational definition of anxiety: Dr. Gomez needs to know the operational definition of anxiety in order to evaluate his patients. So this isn't the correct answer either.
dr. gomez will not know which patients actually received the drug: This matches half of the definition for a double blind study and is correct. What's missing it the indication that the patients also don't know if they're receiving the drug of the placebo. But there's nothing technically wrong with this option and there's obvious errors with the other 3 options, so this is the best choice.
