Which of the following is a characteristic of blood type ab+?
has antibodies a and b in the blood plasma
has antigens a and b on the red blood cells
has no rh antigen on the red blood cell
has antibodies o in the blood plasma

The answer is: has antigens A and B on the red blood cells

Human blood contains either:
A antigens (A)
B antigens (B)
A and B antigens together (AB)
none of these antigens (O)

corresponding to the A, B, AB, and O blood types, respectively.

Antigens are molecules that can cause an immune response to be mounted against them in the form of antibodies. Antigen is an abbreviation for 'antibody generator'.

In the case of the ABO blood groups, the antigens are sugars on the red blood cells.

Antibodies are complex proteins made by certain immune cells, and these proteins have the ability to bind foreign material or microbes, which can block their harmful action and label them for quicker elimination.

Example:
When a person has a blood type with a particular antigen (e.g. A), he or she won't produce antibodies against that blood type, so they are safe to receive blood from individuals with similar blood types (won't be considered foreign). They can also receive blood from blood type O donors because they don't have any antigens that would cause an immune response. 

But this person with blood type A, has antibodies against blood type B (to defend oneself from foreign material), so if they were transfused type B blood in this case, they would experience a dangerous immune reaction as the antibodies would bind the B antigens of the B blood type red blood cells.

Blood type O individuals are universal donors = can donate blood to anyone.

Blood type AB individuals are universal receivers = can receive blood from any group.