He works by fits and starts in the office. Here the underlined idiom means-

The correct answer is- ক) Irregularly.

In / By fits and starts:
- English Meaning: inconsistent, irregular, or intermittently, like doing something sporadically.
- Bangla Meaning: অনিয়মিত ভাবে ঘটা।

Example Sentence:
1. Nothing worthwhile can be accomplished by fits and starts.
2. Martina has been working on her master's thesis by fits and starts.

Source:
1. Merriam-Webster dictionary.
2. Accessible Dictionary.
3. Cambridge Dictionary.