The indefinite article ('a' or 'an') is used before a noun which does not refer to any particular person or thing. It is sometimes used to mean the number 'one'. The definite article, 'the', is used before a noun which refers to a particular person or thing. It refers to a person or thing which is unique or 'one of its kind'. It is also used when a single common noun represents a whole class. 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.
The choice between the indefinite articles, 'a' and 'an' is determined by the sound. Before a word beginning with a vowel sound, 'an' is used. Before a word beginning with a consonant sound, 'a' is used. The noun, 'tonic' begins with a consonant town 't'. The noun 'exercise' begins with a vowel sound, 'e'. Hence, we use 'a' before 'tonic' (a tonic) and 'an' before 'exercise' (an exercise).
The words, 'weak' and 'healthy' are adjectives. However, they are also sometimes used as nouns to refer to the entire class of people who are 'weak' and 'healthy' respectively. Hence, the definite article, 'the' is used before both these nouns.
Option C correctly captures the use of the articles as explained above. Hence, this option is correct.
In options A, B and D, the articles used are incorrect. Hence, these options are incorrect.