5090.4 × 438.7 × 0.2131 is equal in value to:

Concept: shifting a number's decimal point k places to the right multiplies its value by 10k (shifting it left divides by 10k), while the digit sequence itself never changes. So every factor can be written as a fixed digit-mantissa times a power of ten, and for a product of several such factors, the product's own power-of-ten scale is just the sum of each factor's exponent — even though every option below reuses the same three digit-groups 50904, 4387 and 2131.

1. Write each factor of 5090.4 × 438.7 × 0.2131 in mantissa × 10exponent form: 5090.4 = 5.0904 × 103, 438.7 = 4.387 × 102, and 0.2131 = 2.131 × 10-1.

2. Add the exponents to get the expression's overall scale: 3 + 2 + (-1) = 4, so the value equals (5.0904 × 4.387 × 2.131) × 104.

3. Check each option the same way, writing its factors as mantissa × 10exponent and adding the exponents: 50.904 × 4387 × 2.131 gives 101 × 103 × 100 = 104; 509.04 × 438.7 × 213.1 gives 102 × 102 × 102 = 106; 5.0904 × 4.387 × 213.1 gives 100 × 100 × 102 = 102; and 5.0904 × 43.87 × 21.31 gives 100 × 101 × 101 = 102.

4. Only the option whose exponent sum equals the required 104 reproduces the same value as the original expression — that is 50.904 × 4387 × 2.131.

Cross-check by direct multiplication: 5090.4 × 438.7 ≈ 2,233,158.48, and 2,233,158.48 × 0.2131 ≈ 475,886.07. Computing 50.904 × 4387 × 2.131 directly gives the same value, ≈ 475,886.07, confirming the match independently of the exponent method.

So 5090.4 × 438.7 × 0.2131 is equal in value to 50.904 × 4387 × 2.131.