Hello!
You forgot the alternatives!!
Here we have:
a. thymus
b. spleen
c. tissues
d. lymphatic ducts
The only structure listed above that is not a specific part of the adaptive immune system is the letter c) tissues. The tissues are a group of cells that do the same job: connective (maintain the organs in their place), muscle (produce movement within internal organs), nervous (sensory input, control of homeostasis etc), and epithelial (protection).
Hugs!