Change the following statement into passive voice:
Those who worked hard seldom obtained good marks.
a. Good marks were seldom being obtained by those who worked hard.
b. Good marks are seldom obtained by those who worked hard.
c. Seldom had good marks been obtained by those who worked hard.
d. Good marks were seldom obtained by those who worked hard.

Hint: Read the given sentence and understand what it means. Now, try to convey the same sentence to a third person. Go through the options and choose the one which matches or is the closest to the sentence you just framed. Make sure that the sentence is grammatically sound.

Complete answer: Let us first understand what active and passive voices
are. Active voice means that the given sentence has a subject that performs its verb. In short,
this means that the subject is visible or is present in the sentence.
Passive voice means that a subject is in place of an object, i.e. a recipient of a verb’s
action. This means that the subject is not outrightly seen. It is usually there but is
considered invisible. At the end of a passive voice sentence, to make the subject seen,
you just add ‘by someone’. This becomes the default subject.

In order to convert the active voice into passive voice, all you need to do is change the
order of the parts of speech. So, the ‘subject + verb + object’ in the active voice changes to
‘object + verb + (subject)’ in the passive voice.
Now, we know what we are looking for. Let’s look at the given options:
a. Good marks were seldom being obtained by those who worked hard. - This sentence is in the passive voice as the order has been reversed. It has a completely different tense. So, this is not the required answer. So, this is not the correct option.
b. Good marks are seldom obtained by those who worked hard. - This sentence is in the passive voice as the order has been reversed. It has a completely different tense. So, this is not the required answer. So, this is not the correct option.
c. Seldom had good marks been obtained by those who worked hard. - This sentence is not constructed correctly and it is in the wrong tense. This is not the required answer. So, this is an incorrect option.
d. Good marks were seldom obtained by those who worked hard. - This sentence is in the passive voice, as the order of the parts of speech has been reversed. The tense is correct. This is the required answer. So, this is the correct option.
Therefore, option d. is correct.

Note: In active voice, the subject performs the action, however, in the passive voice, the subject is placed at the end of the sentence. It is not outrightly visible, nor is it a command.