Concept:Subject-verb agreement requires a plural verb for the plural noun "oxen", and the time phrase "since morning" shows an action that began in the past and is still continuing.
Explanation:The word "oxen" is the plural form of "ox", so the verb must use the plural auxiliary "have", not "has" or "is"/"are".
The expression "since morning" tells us that the action started earlier and is still happening now.
The present perfect continuous tense is formed with "have/has been" plus the present participle.
Therefore, the correct verb phrase is "have been grazing".
The other options are incorrect because "is being grazed" and "are being grazed" are passive forms, and "has been grazing" uses the wrong singular auxiliary.
Answer:A. have been grazing