Guanine
Step-by-step explanation:
it's not Adenine you've been given Adenine before in the diagram
the DNA ladder is composed of phosphates and deoxyribose sugars arranged linearly and alternatively.
the two chains are also referred to as complementary strands of DNA since one is the exact opposite of the other. Guanine is the opposite of Cytosine and Adenine is the opposite of Thymine. Thus, if Guanine is present on a Complementary DNA Strand, it is Cytosine that will occupy the same position on the other strand and bind with it. Similarly, Adenine will bind with Thymine.