In a DNA double helix an adenine of one strand always pairs with a(n) of the complementary strand, and a guanine of one strand always pairs with a(n) of the complementary strand.

adenine pairs with Thymine  and guanine always pairs with cytosine respectively

Explanation:

In DNA nucleotide subunits, there are four nitrogenous bases:

Each of these bases can be divided into two categories: purine bases and pyrimidine bases.

Adenine and guanine are examples of purine bases. This means their structure is a nitrogen-containing six atom ring joined with a nitrogen-containing five atom ring that share two atoms to combine the two rings.

Thymine and cytosine are examples of pyrimidine bases.

Note that  RNA replaces thymine with a different pyrimidine base called uracil (U).

The complementary base pairing rule, Chargaff's rule states that DNA base pairs are always adenine with thymine (A-T) and cytosine with guanine (C-G). A purine always pairs with a pyrimidine and vice versa. However, A doesn't pair with C, despite that being a purine and a pyrimidine.

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