In which sentence are all periods used correctly?
a. the 5 ft plank was too short and needed to be recut.
b. mr. langan requested a wake-up call for 9: 00
a. m. on saturday.
c. why was there such noise at 6: 00 am?
d. dr. vaughn gave mrs vaughn a pearl necklace for their tenth anniversary.

Ill review each sentence and find their mistakes.

A. The 5 ft plank was too short and needed to be recut.
In this sentence, "ft" needs a period at the end so therefore this sentence isn't correctly punctuated. (a is not the answer)

B. Mr. Langan requested a wake-up call for 9:00 a. M. on Saturday.
In this sentence, the a.m is punctuated incorrectly so this isn't correct either. (b is not the answer)

C. Why was there such noise at 6:00 AM?
Here, it is punctuated correctly. Looks like it is the correct answer.

D. Dr. Vaughn gave Mrs Vaughn a pearl necklace for their tenth anniversary.
In this sentence, "Mrs." didn't have a period at the end (d is not the answer).

So the answer was found and it is C because it is punctuated correctly.  :)