The preposition 'by' indicates that the statement is written in the passive voice. In addition, in the supplied sentence, the subject 'the grocer' has become the object.
To clarify the statement in the simple present tense in the passive voice, we use 'is/am/are' with the past participle form of the verb.
Option A - We can't use 'is' because the topic is plural ('sugar and tea'). If the subject is plural, we need the verb in the plural form, according to subject-verb agreement. 'Sold' is the verb's past participle and can be used with 'are' rather than 'is'. As a result, option A is inaccurate.
Option C - If the subject is plural, we need the verb in the plural form, according to subject-verb agreement. Because the provided sentence has a plural subject, we cannot use 'is' because it is the single form of the verb. In addition, the verb in the past participle form of the sentence/action in the simple present tense is required in the passive voice. As a result, option C is inaccurate.
Option D - Because the supplied sentence has a plural subject, we can use 'are' here. 'Sell' is the original form of the verb, and we need the past participle form. 
Option B - 'Are' can be used with the plural subject here, and sold is the past participle form of the verb sell.' As a result, option B is the correct answer.
'The grocer sells sugar and tea.'