The correct answer is: A. Small chord
A chord of a curve is a line segment that joins two points on the curve. The peg interval is the distance between two consecutive points on the curve. A small chord is a chord that is shorter than the peg interval.
Option B, sub-chord, is not a correct answer because it is not a standard term in mathematics. Option C, normal chord, is not a correct answer because it does not specify that the chord is shorter than the peg interval. Option D, short chord, is not a correct answer because it is not a standard term in mathematics and it is not clear whether it refers to a chord that is shorter than the peg interval or a chord that is shorter than some other reference chord.