Select a suitable option to complete the sentence:
Whenever Arthur was angry, he _____ walk out of the room.
A) could
B) would
C) can
D) may

Hint: An Auxiliary or Modal Verb is one that helps other verbs to form other Tenses than the Simple Present or Simple Past. Thus, they are also known as helping verbs. Auxiliaries that help to express conceptions of the mind are called Modal Auxiliaries.

Complete answer:
A modal verb is a sort of action word that is utilized to indicate modality– that is probability, capacity, authorization, demand, limit, ideas, request, commitment, or exhortation. These verbs show us the attitude of the addresser to what is being said or done.

Let us analyze the given options:
Option A) could: The above option is incorrect as ‘Could’ is used to express an ability that existed in the past, to express possibility or uncertainty or something dependent or unreal conditions and to ask polite questions.

Option B) would: The above option is correct as ‘Would’ is the past tense of ‘will’ and is used to express uncertainty of will that may happen in the future. It also used to talk about habitual actions in the past, but not to talk about past states. Thus, it will fit in the blank in the given sentence.

Option C) can: The above option is incorrect as ‘Can’ is used to express ability or to give permission (but often ‘may’ is used for this purpose).

Option D) may: The above option is incorrect as ‘May’ is used when you are unsure of what you are saying or to express a possibility, to ask permission and to express a wish. It is also used in subordinate clauses that express purpose.

Therefore the correct answer is option ‘B’.

Note: Three basic rules to follow while using Auxiliaries are:
- Use the modal or auxiliary verb as it is. Don't change its form and convert it into the present, future, or past forms.
- Use the base or root form of the verb after a modal. Don't use “to” or the full infinitive verb “to”.
- While using modals in the negative form, use them only with “not” after the modal verb.