Answer:
Change in velocity and direction over a specific period of time.
Explanation:
In physics, acceleration can be defined as the rate of change of the velocity of an object with respect to time.
This simply means that, acceleration is given by the subtraction of initial velocity from the final velocity all over time.
Hence, if we subtract the initial velocity from the final velocity and divide that by the time, we can calculate the acceleration of an object.
Mathematically, acceleration is given by the equation;


Where,
a is acceleration measured in 
v and u is final and initial velocity respectively, measured in 
t is time measured in seconds.
Hence, the types of changes in motion that cause acceleration is a change in velocity and direction over a specific period of time.
Sound level at distance of 15 m is given as 20 dB
so intensity at this distance is given as



now if we move closer to some some distance the sound level is now 50 dB
now the intensity is given as



now we know that



so now the distance from friend must be 47 cm
B - A theory seems to be the closest