Explanation:
Determiners are words that come before nouns and provide information about quantity, possession, or specificity. In this sentence, we need to choose the appropriate determiner to complete the sentence correctly.

Options:
a) Most
b) Most of
c) Either could be used here

Understanding the sentence:
The sentence states that "the shops are closed on Sundays." Since it is referring to a specific group of shops, we need to use a determiner that indicates a specific quantity of the shops that are closed on Sundays.

Explanation of the options:
a) Most: "Most" is a determiner that indicates a large amount or quantity. However, it does not provide the necessary specificity required in this sentence. It does not specify which shops are closed on Sundays, but rather just indicates a general majority.

b) Most of: "Most of" is a determiner that indicates a specific quantity or amount of a larger group. It provides the necessary specificity required in this sentence. By using "most of," we are specifying that a majority of the shops are closed on Sundays.

c) Either could be used here: This option suggests that both options a) and b) could be used interchangeably. However, in this sentence, we need to use a determiner that specifies a specific quantity or amount of the shops that are closed on Sundays, so option b) is the correct choice.

Conclusion:
The appropriate determiner to complete the sentence is "Most of" (option b) as it provides the necessary specificity by indicating a specific quantity or amount of the shops that are closed on Sundays.