Select the most appropriate option to fill in the blank.

Their new house costed them _______ hundred and fifty-six thousand pounds.

a

The most appropriate option to fill in the blank is: a

The corrected sentence would be: "Their new house costed them a hundred and fifty-six thousand pounds."

Explanation:
In this context, "a" is used to refer to a singular, unspecified amount or quantity of money. It indicates that the cost of the house is one specific amount, which is "hundred and fifty-six thousand pounds." Therefore, "a" is the appropriate article to use in this sentence.