Let's go through each part of the sentence completion exercise and identify the correct choices:
It was the ______ book I've ever read. The best choice is (b) most exciting because we are comparing one book to others, meaning it stands out the most in terms of excitement.
She ______ any free time today. The best choice is (a) doesn't have because we are describing a present tense situation where she lacks free time.
Are these the suitcases ______ you used? The correct choice is (b) which because we use "which" when referring to things, like suitcases, in a question.
My old flat was ______ than this one. The correct choice is (b) bigger as we are comparing sizes of two flats, so a comparative form of the adjective 'big' is needed.
They ______ unhappy when the rain started. The correct choice is (a) looked because "looked" is used to describe their appearance, whereas "looked like" would require a comparison with a noun.
You shouldn't ______ that. It isn't nice. The correct choice is (a) say because 'say' is the verb required after 'shouldn't' without the 'to.
You ______ do it today. Wait until tomorrow. The correct choice is (b) don't have to as it suggests that there is no necessity to perform the task today.
Has Phil ______ met anybody famous? The correct choice is (a) ever which means at any time in the past, suitable for an open-ended question like this.
Always ______ the receipt. The correct choice is (a) check as 'check' is a command or instruction to always verify the receipt immediately.
Each choice was made by considering the grammatical structure and meaning required in the context of each sentence. These types of exercises are common in English classes to help students practice grammar and understand sentence structures better.