To determine the correct option, we must first understand what an algal bloom is and the characteristics of oligotrophic lakes.

An algal bloom occurs when algae grow rapidly, often due to an increase in nutrients like nitrogen and phosphorus. These blooms can cause problems by depleting oxygen in water bodies, leading to dead zones where aquatic life cannot survive.

An oligotrophic lake is characterized by a low concentration of nutrients and consequently low productivity in terms of aquatic life. Such lakes have clear waters, with minimal plant life and algae because of the limited availability of nutrients.

Given these characteristics, let's examine why the correct answer is "very rare":

• Oligotrophic lakes have low nutrient levels, which do not support the high growth rates needed for algal blooms.
• Therefore, algal blooms are uncommon due to this scarcity of essential nutrients.

Let's evaluate the options:

• Very frequent: Not correct. Algal blooms need high nutrient levels, which are absent in oligotrophic lakes.
• Frequent: Not correct for the same reason; nutrient limitation doesn't support frequent blooms.
• Very rare: Correct. The scarcity of nutrients makes algal blooms a rare occurrence in oligotrophic lakes.
• Widespread: Not correct. Algal blooms cannot be widespread in oligotrophic lakes due to the lack of nutrients.

Thus, the correct answer is that algal blooms in oligotrophic lakes are very rare.