The mutations that arise as a result of natural events are called spontaneous mutations. They can be transmitted from one generation to the other.
The spontaneous mutation is mostly caused due to tautomerism, depurination, deamination, and slipped strand mispairing.
The nonsense mutation is a point mutation, in which the premature stop codon occurs in the sequence of DNA. Hence, the option (b) is not correct.
An indel mutation is a frameshift mutation, which involves removal or addition of nucleotides from nucleic acids. Hence, the option (c) is not correct.
The nonsense mutation is a point mutation, in which the premature stop codon occurs in the sequence of DNA. The spontaneous mutation is caused by slipped strand mispairing but not by indel mutation. Hence, the option (d) is not correct.
The tobacco exposure is the leading cause of cancer in respiratory tract. The p53 mutation mostly causes lung cancer. The mutation of p53 is caused by smoking and this mutation is a spontaneous mutation.
Hence, the correct option is a.