Final answer:
For anchoring a boat in 20 feet of water, one should use a scope ratio of 3:1 to 5:1, thus requiring 60 to 100 feet of anchor line; the best choice is 60 to 80 feet for stability.
Step-by-step explanation:
When anchoring a boat, it is important to consider the depth of the water to ensure that the boat remains stationary. The general rule of thumb for anchoring is to use a scope ratio of 3:1 to 5:1, which means you should have 3 to 5 times the length of anchor line compared to the depth of the water. In this case, the water is 20 feet deep. So, using the 3:1 ratio, you would need at least 60 feet of line (20 feet deep × 3). However, for greater stability, particularly in changing conditions, it may be advisable to use a 5:1 ratio, which would require 100 feet of line (20 feet × 5). Therefore, the best choice would be b) 60 to 80 feet, as it provides a safe and practical scope range for most anchoring situations.