The volume of a barrel of oil is calculated as 257166cm3 round this figure to the nearest hundred thousand

Remember, the hundred thousand place is where the 2 is located. Continuing on, the ten thousand place is 5.

Now, the rule of rounding is if the number before the number we're trying to round is less then 5 then that particular number we're trying to round stays the with the previous numbers being zeros. 

However, if the number before the number we're trying to round is greater or equal to 5, then that particular we're trying to round goes up a number. (In other words, it goes up a number value.)

According to this questions, the second rule we have meets the criteria.

So the answer would be 300000cm^3.

If you have any more questions, please place it on the comments below.

Hope this helps!