Final answer:
Cytosine (C) is the most mutagenic nitrogenous base during spontaneous deamination, as its conversion to uracil leads to a base substitution in the DNA sequence.
Step-by-step explanation:
The nitrogenous base that is most mutagenic during spontaneous deamination is cytosine (C). Spontaneous deamination is the removal of amino groups from the nitrogenous bases. For cytosine, this removal converts it into uracil, which then pairs with adenine instead of guanine. This leads to a base substitution from a G-C pair to an A-T pair during DNA replication. The original pairing of cytosine is with guanine, but after deamination and subsequent replication, the pairing changes, leading to a mutation which, if uncorrected, results in the mispairing becoming permanent. In contrast, deamination of adenine or guanine produces hypoxanthine and xanthine, respectively, which are more likely to be recognized and corrected by the cellular DNA repair mechanisms.