Mr. Lamb is a retired man who keeps himself busy in various ways. Let's explore each option to understand how he spends his time.

a) Reading books:
One way Mr. Lamb keeps himself busy is by reading books. Reading can be a great way to pass the time and engage the mind. It allows one to escape into different worlds, learn new things, and broaden one's perspective. Mr. Lamb might enjoy a variety of genres, such as fiction, non-fiction, or biographies, depending on his interests.

b) Chatting with people:
Another way Mr. Lamb stays occupied is by chatting with people. Socializing is an important aspect of human life, and it helps to maintain connections and build relationships. Mr. Lamb may engage in conversations with friends, neighbors, or family members either in person or through phone calls, text messages, or video chats. This social interaction can provide mental stimulation and a sense of belonging.

c) Pulling down the ripe crab apples of his garden:
Mr. Lamb also keeps himself busy by pulling down the ripe crab apples from his garden. This suggests that he might have a garden where he grows fruit trees, specifically crab apple trees. Harvesting the ripe fruits not only helps him utilize the produce but also keeps him physically active. Gardening can be a therapeutic and rewarding activity that allows individuals to connect with nature and enjoy the fruits of their labor.

d) All these:
The correct answer is option 'D' because all the mentioned activities are ways in which Mr. Lamb keeps himself busy. Reading books, chatting with people, and tending to his garden by pulling down the ripe crab apples are all different ways for Mr. Lamb to pass his time and engage in various activities.

In conclusion, Mr. Lamb keeps himself busy by reading books, chatting with people, and tending to his garden. These activities provide mental stimulation, social interaction, and physical activity, ensuring that he leads an active and fulfilling retired life.