Answer:
The correct answer is the option D: the MTBF distribution of products, machines, or processes that have "settled in".
Explanation:
To begin with, the term of "normal distribution" refers to statistic model used in the field of probabilities and the most common characteristics are that they have a standard deviation of 1 and are all symmetrical. In the proper graphic the form of a normal distribution will be the one of a curve bell. Therefore that it is regularly seen in as an appropiate model of MTBF distribution regarding maintenance of products, machines or processes. Moreover, this distribution also known as Gaussian is motivated by the Central Limit Theorem.