Badi Chaudas is a vrata/fast day for many Jains, and on such days people choose carefully what they eat. Whether you can eat biscuits depends on the vow you are observing that day.

• If you are observing a stricter upvas on Badi Chaudas, you should prefer simple, accepted fasting foods or abstain from solid food after a certain time. In that case, biscuits would not be appropriate.
• If you are eating (parvas or a lighter vrat) and you want to have biscuits, they should be Jain-approved: no eggs, no gelatin, no animal fat, and no ingredients that violate your vow. Many people avoid eggs and gelatin and prefer items prepared with pure vegetarian ingredients.
• A safe check: choose biscuits that are clearly labeled as Jain-friendly or make them at home with vegetable (no animal-derived) ingredients. Also be mindful of any dairy if your tradition avoids milk-based products on certain days.
• Note: Digambar and Shwetambar communities sometimes have different practices for specific fast days. If you follow a particular tradition, follow the guidelines given by your guru or local temple.

If you’d like, I can guide you to a Jain-specific page with more detailed notes on Badi Chaudas and vrat rules from your tradition.