The problem can be solved step by step:

1. Determine the total number of sacks needed:

   • The baker is making 7 batches of doughnuts.
   • Each batch requires 15 sacks of flour.
   • So, the total number of sacks needed is: $7 \times 15 = 105 \, \text{sacks}.$

2. Determine how many crates are needed:

   • Each crate contains 4 sacks of flour.
   • To find how many crates are needed for 105 sacks, divide the total number of sacks by the number of sacks per crate: $\frac{105}{4} = 26.25.$
   • Since we can't order a fraction of a crate, the baker would need to round up to the next whole number, which means ordering 27 crates.

Thus, the baker should order 27 crates of flour, which corresponds to option D.

Would you like further details or clarification?

Here are 5 related questions you might explore:

1. What would happen if each batch needed fewer sacks of flour?
2. How would the number of crates change if each crate could hold more sacks?
3. What if the baker made more than 7 batches? How would the calculation change?
4. Can we generalize the formula for the number of crates if the sacks per batch or crates vary?
5. How does rounding up to the nearest whole number affect cost estimation in such problems?

Tip: Always double-check if a problem requires rounding up when dealing with practical units like crates or boxes!