Which of the following word is singular?

The principal took the round in the college. Here 'Round' is a/an- Which one of the following is not a plural number? Shahed did not join the army. Here the word army is- They tell us a tale about a tail. The word 'tale' is - What is the plural number of 'spectrum'? What is the plural form of the word louse? A herd of cattle is passing. The underlined word is a/an- I had a talk with him. Here 'talk' is a/an We need to buy some new--- Which one is plural-