The correct answer to the question is option 3 - High levels of taxation.

During the Bengal Famine of 1769 - 70, several factors contributed to the worsening of the situation. One of the major factors was the high levels of taxation imposed on the local population. The British East India Company, which had control over Bengal at the time, imposed heavy taxes on the agricultural produce of the region.

These taxes put a significant burden on the farmers and limited their ability to produce and store food. As a result, the food supply in the region was severely affected, leading to widespread scarcity and hunger. The high levels of taxation also discouraged trade and limited the availability of food from other regions, further exacerbating the famine.

It is important to consider that while the other options - locust attacks, rat infestation, and smallpox epidemic - may have played a role in aggravating the famine, the key factor mentioned in option 3 - high levels of taxation - had a more direct impact on the availability and affordability of food for the local population.