Answer :
The easiest way to tell is to line them all up in a column.
A - C - T - G - G - A - T
A - C - T - G - G
A - G - C - T - G - G - T
A - C - T - T - G - A - T
A - C - G - G - C - T - T
The shortest one will be a deletion. This is answer A. The last two letters in the sequence (A and T) have been removed.
A - C - T - G - G - A - T
A - C - T - G - G
A - G - C - T - G - G - T
A - C - T - T - G - A - T
A - C - G - G - C - T - T
The shortest one will be a deletion. This is answer A. The last two letters in the sequence (A and T) have been removed.