To solve this problem, we can use trigonometric identities and simplify the given expression step by step. Let's break down the solution into different parts:

Given expression: cos8x * 2cos6x * cos4x

1. Simplify the expression sin x * sin 2x = 1:
- We know that sin 2x = 2 * sin x * cos x.
- Substituting this in the given expression, we have: sin x * (2 * sin x * cos x) = 1.
- Simplifying further, we get: 2sin^2x * cos x = 1.

2. Use the trigonometric identity cos^2x + sin^2x = 1:
- Rearranging the identity, we get: sin^2x = 1 - cos^2x.
- Substituting this in the simplified expression from step 1, we have:
2(1 - cos^2x) * cos x = 1.

3. Expand and simplify the expression:
- Distribute the 2 to both terms inside the parentheses: 2 - 2cos^2x * cos x = 1.
- Simplify further: 2cos^3x - 2cos^2x = 1.

4. Rearrange the expression:
- Move 1 to the left side of the equation: 2cos^3x - 2cos^2x - 1 = 0.

5. Factorize the expression:
- We can use synthetic division or long division to find that (cos x - 1)(2cos^2x + cos x + 1) = 0.

6. Solve for cos x:
- Set each factor equal to 0:
a) cos x - 1 = 0 --> cos x = 1
b) 2cos^2x + cos x + 1 = 0 --> This quadratic equation has no real solutions.

7. Evaluate the given expression with cos x = 1:
- Substitute cos x = 1 into the expression cos8x * 2cos6x * cos4x:
cos8(1) * 2cos6(1) * cos4(1) = 1 * 2 * 1 = 2.

Therefore, the correct answer is option 'D' (2).