X rounded to tens place is 20. ( X > 15 and we want to round to tens place)
X rounded to the nearest tenth is 15.2 ( 15.246 rounded up)

So 20 * 15.2 = 304

Hence Option (D) is our answer.

Best,
Gladi
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15.246 rounded to the nearest tenth is 15.2. You should take into the account only the first dropped digit. So, rounding is not cumulative.

Rounding is simplifying a number to a certain place value. To round the decimal drop the extra decimal places, and if the first dropped digit is 5 or greater, ROUND UP the last digit that you keep. If the first dropped digit is 4 or smaller, ROUND DOWN (keep the same) the last digit that you keep.

Example:
5.3485 rounded to the nearest tenth = 5.3, since the dropped 4 is less than 5.
5.3485 rounded to the nearest hundredth = 5.35, since the dropped 8 is greater than 5.
5.3485 rounded to the nearest thousandth = 5.349, since the dropped 5 is equal to 5.

So, according to the above 8.35y rounded to the nearest tenth will be 8.4 irrespective of the value of y.

For mote on this check the following posts:
Math: Number Theory
Rounding Rules on the GMAT: Slip to the Side and Look for a Five!

3. Fractions, Decimals, Ratios and Proportions

• Theory
  GMAT Shortcut: Adding to the Numerator and Denominator
  Rounding Rules on the GMAT: Slip to the Side and Look for a Five!
  How to Identify Terminating Decimals on the GMAT
  Rounding Up Some Official GMAT Questions!
  Terminating Decimals in Data Sufficiency on the GMAT
  GMAT Quantitative: Ratio and Proportions
  Ratios in GMAT Data Sufficiency
  Why You Should Convert Fractions to Decimals on the GMAT
  Use This Strategy for Fractions and Save Time on GMAT Test Day
  Variations On The GMAT - All In One Topic
  
  • Questions
    Terminating and Recurring Decimals
    Units digits, exponents, remainders problems
    DS Questions
    PS Questions

For more:
ALL YOU NEED FOR QUANT ! ! !
Ultimate GMAT Quantitative Megathread


Hope it helps.
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