Fill in the blank with an appropriate preposition:They decided to invest in a diversified portfolio to provide _____ market volatility.

• Provide against (phrasal verb): 
English Meaning: to make preparations to deal with something bad or unpleasant that might happen in the future
Bangla Meaning: সতর্কতামূলক ব্যবস্থা নেওয়া; সংস্থান করা; দুর্দিনের জন্য সঞ্চয় করা। 
Example: We have to provide against rainy days. 

এছাড়াও, 
• Provide with - সরবরাহ করা। 
Example: The students were provided with papers. 
• Provide for -  যথাযথ প্রস্তুতি গ্রহণ করা; প্রতিপালন/ভরণপোষণ করা; সংস্থান/সৎকার করা। 
Example: I have many dependants to provide for.

Complete Sentence: They decided to invest in a diversified portfolio to provide against market volatility.

Source:
1. Cambridge Dictionary. 
2. Accessible Dictionary by Bangla Academy.