Step 1: {Young's modulus dependency}
\[ Y = \frac{{Stress}}{{Strain}} \] Step 2: {Effect of change in length and area}
Young's modulus is an intrinsic material property, unaffected by variations in length or cross-sectional area. Consequently, the correct designation is \( Y \).
A particle is moving in a straight line. The variation of position $ x $ as a function of time $ t $ is given as:
$ x = t^3 - 6t^2 + 20t + 15 $.
The velocity of the body when its acceleration becomes zero is: