What is the correct word to complete this sentence: ______ est vraiment intéressant?

The correct word to use in this sentence is 'cela' which translates to 'that' in English. 'Ce' translates to 'this', 'ça' is more informal and corresponds to 'that' in English, and 'cet' is used before masculine nouns that begin with a vowel or a silent 'h'.