The correct answer is option 2: Blood.

Tissue refers to a group of similar cells that work together to perform a specific function in the body. Blood is a tissue because it is made up of different types of cells that work together to carry out functions like transporting oxygen, nutrients, and hormones throughout the body, as well as removing waste products. Blood consists of red blood cells, white blood cells, platelets, and plasma.

Option 1, Brain, is an organ made up of different types of tissues, including nervous tissue, but it is not an example of a single tissue.

Option 3, Liver, is also an organ made up of different types of tissues, including hepatocytes, bile ducts, and blood vessels, but it is not a single tissue by itself.

Option 4, Stomach, is an organ made up of different types of tissues, including muscular tissue and epithelial tissue, but it is not a single tissue.

Therefore, the correct answer is option 2, Blood, as it is a specific type of tissue made up of different types of cells working together to perform a common function.