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.
The first blank shall take 'the' as a particular item's (sugar's) price is being talked about while the second and third blank shall take 'a' as the second blank is followed by an expression of price( a penny a kilo).
The correct answer is A) The /a/a.
The price of sugar has risen by a penny a kilo.