Hint: The Upanishads were written in India. The time period of its writing probably spans between 800 BC to 500 BC. It is a part of the Vedas and is written in Sanskrit.
Complete answer: Let us have a look at the options:
Option a: Although both the Vedas and the Upanishads are central to Hinduism, we cannot ascribe it a complete religious identity. It also had thoughts from Jainism and Buddhism. Thus, it is a wrong option. Option b: Yoga does form one of the many constituents of the Upanishads. However, it is not the whole of it. Hence, it is a wrong option. Option c: Like the last option, laws do form a part of the entire book and is not the entire identity of the book. Thus, it is a wrong option. Option d: The Upanishads mainly deal with the philosophical concepts both Hinduism, Jainism and Buddhism. The central discussion is indeed around philosophy. Thus, this is the right option.
Hence, the correct answer is option d.
Note:The Upanishads are very comprehensive and extensively written. It thus can become hard to draw lines of what it really is about. But the central theme is always philosophical and hence, we can say they are books on philosophy.