**Explanation**

The letter that is silent in the word "walk" is **b) L**.

**Phonetic Pronunciation**
The word "walk" is pronounced as /wɔːk/. The initial sound is the "w" sound, followed by the "aw" sound as in "ball," and ending with the "k" sound.

**The Silent Letter**
The letter "l" in the word "walk" is silent. This means that it is not pronounced when saying the word.

**Examples of Silent Letters**
Silent letters can be found in many English words. Here are some examples:

1. **Knock** - The "k" is silent.
2. **Wrist** - The "w" is silent.
3. **Debt** - The "b" is silent.
4. **Island** - The "s" is silent.

**Reason for Silent "L"**
The reason why the "l" in "walk" is silent is due to the phonetic evolution of the English language. Over time, the pronunciation of certain words changed, but the spelling remained the same. In this case, the "l" in "walk" used to be pronounced in older forms of English, but the pronunciation changed while the spelling remained unchanged.

**Common Misconception**
It is important to note that the "k" in "walk" is not silent. Some people may mistakenly think that the "k" is silent because it is not pronounced strongly, but it is still necessary to say the "k" sound when pronouncing the word.

**Conclusion**
In the word "walk," the letter "l" is silent. This means that it is not pronounced when saying the word. Silent letters can be found in various English words and are a result of the evolution of the English language.