1. I lent him a book. It was written by Daniel Defoe.
> The book  which  I lent him was written by Daniel Defoe
2. A nutritionist is an expert. A nutritionist studies the relationship between foods and health.
>A nutritionist is an expert who  studies the relationship between foods and health.
3. My friend has decided to buy a motorbike. His bicycle was stolen last week.
> My friend whose  bicycle was stolen last week  has decided to buy a motorbike
4. "Who Wants to be a Millionaire?" is a TV show. It is very popular.
=> "Who Wants to be a Millionaire?" is a TV show which is very popular.
5. Yoga is a type of exercise. Many people enjoy yoga.
=> Yoga is a type of exercise which many people enjoy