Read the sentence. Avoidable means
Many bike accidents are avoidable, especially if riders
wear helmets and follow traffic rules.

- hard to avoid ?
- able to avoid
- the process of avoiding
- avoiding afterward

Able to avoid

Explanation:

Breaking down the construction of the word -able means possible and avoid means the ability to prevent. The sentence shows that bike accidents are preventable under certain circumstances. Thus, the other options for the definition would not be the correct interpretation of the word in the sentence.

the correct answer would be:  

a. adverb phrase