The farmer is ___ cutting the crop which had been ripened.

The sentence requires a verb that forms the present continuous tense to describe an ongoing action. The structure for the present continuous tense is 'subject + is/am/are + verb-ing'.

• D. Is: This is the correct auxiliary verb to form the present continuous tense ('is cutting') with the singular subject 'The farmer'. It indicates that the action is happening now.
• A. Was: This would form the past continuous tense ('was cutting'), implying the action happened in the past, which doesn't fit the implied present context.
• B. Has: This would typically be used for the present perfect tense ('has cut') or present perfect continuous ('has been cutting'), neither of which fits directly with 'cutting' alone to describe an ongoing action.
• C. Had: This would form the past perfect tense ('had cut') or past perfect continuous ('had been cutting'), indicating an action completed before another past action.