Which one is in the singular number?

Which one is singular? Walking is good for health. এ বাক্যে walking শব্দটি- Which of the following does not add 'ves' in the plural form? The word 'Gravity' is- Which one of the following is not a plural number? What part of speech is 'Scarcity'? Which of the following word is singular? The principal took the round in the college. Here 'Round' is a/an- A herd of cattle is passing. The underlined word is a/an- Accommodation is-