Let the number of workers be 7x and the number of managers be 2x. The total number of employees is:
\[ 7x + 2x = 45,000 \]
Solving for x:
\[ 9x = 45,000 \Rightarrow x = 5,000 \]
Thus, the number of workers is:
\[ 7x = 35,000 \]
And the number of managers is:
\[ 2x = 10,000 \]
To achieve a ratio of 5 workers to 1 manager, the number of workers should be:
\[ 5 \times 10,000 = 50,000 \]
Thus, the manufacturing plant needs to hire:
\[ 50,000 - 35,000 = 15,000 \]
The number of new workers to be hired is 15,000.
Correct answer: (C) 15,000.