1. The problem asks for the least common denominator (LCD) of the two fractions $\frac{3}{10x^3}$ and $\frac{y}{14x^5}$.\n\n2. The LCD is the smallest expression that both denominators divide into without remainder. It must include all prime factors and variables with the highest powers from both denominators.\n\n3. Factor the denominators:\n- First denominator: $10x^3 = 2 \times 5 \times x^3$\n- Second denominator: $14x^5 = 2 \times 7 \times x^5$\n\n4. For the LCD, take the highest powers of each prime factor and variable:\n- For numbers: $2$ (common), $5$ (from first), $7$ (from second)\n- For $x$: highest power is $x^5$\n\n5. Multiply these together:\n$$\text{LCD} = 2 \times 5 \times 7 \times x^5 = 70x^5$$\n\n6. Therefore, the least common denominator is $70x^5$.\n\nFinal answer: $70x^5$ (Option D).