All of the following combinations are possible in the gametes of an
organism that is AaBb EXCEPT:
a) aB 
b) Ab 
c) ab 
d) aa 
e) AB

Hi operator -

The gametes contain only ONE of the alleles (versions of a gene)
available from a parent.  Gametes are sperm and ova, and they combine
with their opposite (usually) to complete the genetic package and form
an organism.

The options available to our "person" with the AaBb setup are as follows:

AB
Ab
aB
ab

Thus, Answer D (aa) is the one that is not possible - it contains two
alleles for the one gene (A/a) and none for the second (B/b).

Librariankt

aa