Q. Morning walk is . . . . . . . . tonic for . . . . . . . . weak, cure for the sick and . . . . . . . . exercise for . . . . . . . . healthy.

Explanation by: Mr. Dubey

In the given sentence, the word, 'tonic' means 'a medicinal substance taken to give a feeling of vigour or well-being'. There are many 'tonics' in the world. 'Morning walk' is mentioned as one of them. As it refers to 'one of many' tonics, an indefinite article is used before the word, 'tonic'. The word 'exercise' is also used similarly. So, before the words, 'tonic' and 'exercise', an indefinite article is to be used.