I'm pretty sure that it's natural gas.
I believe the answer would be D) kitchen scraps and grass clippings
For example: GGATACCTAGGTAAT
Insertion: GGAT<u>G</u>ACCTAGGTAAT
a nucleotide that was not present before is inserted into the original sequence
Substitution: <u>C</u>GATACCTAGGTAAT
the first nucleotide was substituted in for another (C for G)
Frameshift (basically a insertion or deletion, we'll try deletion here): GATACCTAGGTAAT
the first nucleotide was deleted, causing the reading frame to be shifted (instead of GGA as the first codon, we have GAT)
