To answer this question we need to remember what a frameshift mutation is: a mutation that implies an insertion or deletion of a nucleotide changing the letters that integrate each codon.
Original sequence: ATCTAGGTACTA
Comparison option by option
A T C T A G G T A C T A option A
A T - T A G G T A C T A
A T C T A G G T A C T A option B
A T T T A G G T A C T A
A T C T A G G T A C T A option C
A T C A T G G A T C T A
A T C T A G G T A C T A option D
G T C T G G G T G C T G
As we can see, options B and C are equal, option D has mutation but not insertion o deletion so does not apply, therefore we have left A, where we can see a deletion at the third base, Therefore the correct answer is option A.