One of the primary functions of blood is to deliver oxygen-rich blood to the body's tissues. This is accomplished by ________________, which contain the oxygen-carrying protein called ___________________.

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Erythrocytes, or red blood cells, ("eryth" meaning red, and "cyte" meaning cell) are the cells found in blood that bind and deliver oxygen. They are able to carry oxygen because of a protein they contain called hemoglobin.  Hemoglobin is an iron-based protein and is actually what gives blood its red pigment.
Platelets are small cells which are also found in blood but have a different job. Their role is to form blood clots and stop excessive bleeding by flocking together when there is an injury to the blood vessel. They do not contain hemoglobin.
Leukocytes are also found in blood, but these are a group of white blood cells that have various specific functions relating to immune function. Leukocytes do not contain myoglobin, which is an oxygen-carrying protein found in muscle, not blood.
Plasma makes up the watery component of blood and serves to help transport various substances throughout the body. The protein albumin is found in plasma and binds substances such as hormones and electrolytes. While this is an important function, oxygen binding and transport is not a function of plasma or albumin.

One of the Primary Functions of Blood Is to Deliver