To solve this problem, we need to consider the given conditions:

1. The word must be 5 letters long.
2. The letter M must always be selected.

Let's break down the solution step by step:

Step 1: Count the total number of letters in the word 'MOISTURE'.
The word 'MOISTURE' has 8 letters in total.

Step 2: Select the letter M.
Since the letter M must always be selected, we have only one choice for the first letter.

Step 3: Select the remaining 4 letters from the remaining 7 letters.
Now, we need to select 4 letters from the remaining 7 letters: O, I, S, T, U, R, E.

To calculate the number of ways to select 4 letters from 7, we can use the combination formula:

nCr = n! / (r! * (n-r)!)

where n is the total number of objects and r is the number of objects to be selected.

In this case, n = 7 (remaining letters) and r = 4 (letters to be selected).

Using the formula, we can calculate 7C4 as follows:

7C4 = 7! / (4! * (7-4)!)
= (7 * 6 * 5 * 4!) / (4! * 3 * 2 * 1)
= 7 * 6 * 5 / (3 * 2 * 1)
= 35

So, there are 35 ways to select the remaining 4 letters.

Step 4: Arrange the selected letters.
Now, we have the letter M as the first letter and the remaining 4 selected letters. We need to arrange them to form different words.

Since the order of the selected letters matters, we need to use the permutation formula:

nPr = n! / (n-r)!

In this case, n = 5 (total letters to be arranged) and r = 5 (total letters).

Using the formula, we can calculate 5! as follows:

5! = 5 * 4 * 3 * 2 * 1
= 120

So, there are 120 ways to arrange the selected letters.

Step 5: Multiply the number of ways to select and arrange the letters.
To find the total number of 5-letter words that can be formed, we need to multiply the number of ways to select the 4 letters and arrange them.

Total number of words = 35 * 120
= 4200

Therefore, the correct answer is option C) 7C3 * 5!.

Since M is always selected, we select 4 other letters from 7.