Select the word or phrase that makes this sentence grammatically correct.

Economics ------- my least favorite subject at school.

This sentence uses the word "too" correctly as an adverb to indicate an excessive degree or extent. In this case, it emphasizes that the weight of the object is too heavy for the speaker to lift.

The other options contain incorrect uses of the word "too":

A. "I have too go for my exercise class now." - The correct word to use here is "to" as a preposition, indicating the direction or purpose of going. It should be "I have to go for my exercise class now."

B. "It is important too spell correctly." - The correct word to use here is "to" as an infinitive marker. It should be "It is important to spell correctly."

D. "We had too kinds of desserts yesterday." - The correct word to use here is "two" as a number indicating quantity. It should be "We had two kinds of desserts yesterday."

Therefore, option C is the correct and grammatically sound sentence.

A. The results suggests the patient has a bacterial infection.

This sentence is incorrect. The subject "results" is plural, so the correct form of the verb should be "suggest" to match the subject. The correct sentence would be "The results suggest the patient has a bacterial infection."

B. The student were excited to have passed their exams.

This sentence is incorrect. The subject "student" is singular, so the verb should be "was" instead of "were" to match the subject. The correct sentence would be "The student was excited to have passed their exams." (Note: "their" should be changed to "his" or "her" to match the singular subject "student."

C. The bottle of tablets has an expired expiration date.

This sentence is correct. The subject "bottle" is singular, so the verb "has" agrees with it

D. Harmony are agreeable features of classical music.

This sentence is incorrect. The subject "harmony" is singular, so the verb should be "is" instead of "are" to match the subject. The correct sentence would be "Harmony is an agreeable feature of classical music."

In this sentence, the word "me" is used as the object of the preposition "to." When multiple people are involved as recipients of an action, the pronoun "me" is used in conjunction with the other person or people.

In this case, "my friend and me ..." are the recipients of the chocolate bar, making "me" the appropriate pronoun to use.

The correct preposition to use in this context is "if" instead of "by."

Therefore, the corrected sentence would be: "Confidentiality should be observed at all times, so the nurse should not discuss clients when the conversation can be overheard by others."

The options A. "should not," B. "be observed," and C. "can be" are used correctly in the sentence. It is only the incorrect use of "By" that makes the sentence grammatically incorrect