. . . . . . . . inflation is defined as . . . . . . . . persistent rise in . . . . . . . . general level of prices.

An indefinite article is used when we are talking generally about something whereas a definite article is used when we are talking about something specific.
The indefinite articles are 'a' and 'an'. An is used before a word starting with a vowel or vowel sound. A is used before a word starting with a consonant or consonant sound.
A definite article is an article that tells the uniqueness of something.
'The' is the only definite article.
There won't come an article in the first blank as 'inflation' is an abstract noun. The second blank shall take 'a' as 'persistent rise ' is a noun complement.
The third blank shall take 'the' as the sentence talks about a specific level of prices (general).