The correct answer is option 2: Monocytes. Monocytes are a type of white blood cell that circulates in the bloodstream and plays a key role in the immune response. They are characterized by their large size, typically measuring 12-20 micrometers in diameter, making them the largest cells in mammalian blood.

Let`s review the other options to understand why they are not the correct answers:

Option 1: Erythrocytes, also known as red blood cells, are the most abundant cells in mammalian blood. However, they are much smaller than monocytes, with a diameter of approximately 7-8 micrometers.

Option 3: Basophils are a type of white blood cell involved in allergic reactions and immune responses against parasites. While they are larger than erythrocytes, they are still smaller than monocytes.

Option 4: Lymphocytes are another type of white blood cell involved in the immune response. They come in different subtypes, including T cells and B cells. While lymphocytes can vary in size, they are generally smaller than monocytes.

To conclude, out of the given options, monocytes are the largest cells in mammalian blood.