This cost me ____ hundred rupees.

English uses the indefinite article 'a' before 'hundred', 'thousand', 'dozen', and 'score' whenever the word stands alone as a full cardinal number (equivalent to 'one hundred', 'one thousand', 'twelve') and is not already quantified by another numeral. The choice between 'a' and 'an' follows the SOUND of the very next word, not its spelling: 'a' goes before a word that begins with a consonant sound, 'an' goes before a word that begins with a vowel sound.

In 'This cost me ____ hundred rupees', nothing before the blank already states a count — there is no 'two', 'five', 'several', or similar numeral — so 'hundred' here stands alone as a complete cardinal number meaning 100. That standalone use requires the indefinite article immediately before it. Because 'hundred' is pronounced with an audible /h/ sound — a consonant sound, not a vowel sound — the required article is 'a', not 'an'.

• 'two hundred rupees' needs no article at all, because the numeral 'two' already quantifies 'hundred' — the blank in this sentence has no such preceding numeral.

• 'an hour' takes 'an' only because its initial 'h' is silent; 'hundred' has a clearly pronounced initial 'h', so the same 'an' does not carry over.

• 'the hundred rupees' would need a specific, already-identified amount that speaker and listener both recognise from earlier context — this sentence sets up no such known amount.

This confirms the article needed here is 'a', giving the grammatically complete sentence 'This cost me a hundred rupees.'