In the initials puzzle 90 D in RA, what complete phrase is being encoded?

Introduction / Context:
This question is another example of a number and letters, or ditloid, puzzle. The code 90 D in RA must be expanded into a familiar mathematical or factual sentence. These puzzles force you to connect symbolic initials with full English phrases drawn from basic school knowledge. In this case, the relevant concept is geometry and the standard measure of a right angle.

Given Data / Assumptions:
- The number 90 is given at the start of the puzzle. - D probably stands for degrees or days, which are common words used with numbers. - RA is likely an abbreviation for a two word phrase beginning with R and A. - The complete sentence should be a well known fact, not a rare statement.

Concept / Approach:
At school, students learn that a right angle in geometry measures exactly 90 degrees. This is a fundamental fact used in triangles, coordinate systems and trigonometry. The phrase 90 degrees in a right angle fits the pattern perfectly: D stands for degrees and RA stands for right angle. It uses basic mathematical vocabulary and is widely known, which makes it an ideal candidate for this type of puzzle. Once that connection is made, the decoding is straightforward.

Step-by-Step Solution:
1. Identify D as degrees or days, and check which choice produces a standard fact. 2. Recall that a right angle is precisely defined as 90 degrees. 3. Map RA to right angle, which is a textbook abbreviation. 4. Combine these elements into the sentence 90 degrees in a right angle. 5. Confirm that this is a familiar and accurate geometric statement.

Verification / Alternative check:
If you test each option by compressing it back into initials, option B matches exactly: 90 degrees in a right angle becomes 90 D in RA. In contrast, 90 days in rainy autumn would compress to 90 D in RA but rainy autumn is not a standard time period taught in general knowledge. The other options involving road atlases, academic terms and random arrays are not standard facts that most students would immediately recognise. Because geometry clearly provides the specific well defined fact at 90 degrees, this verification supports option B as the intended decoding.

Why Other Options Are Wrong:
Option A invents a phrase about rainy autumn that is not a fixed unit or notion in school education. Option C, D and E refer to arbitrary collections of divisions, days or digits that could be any number; 90 has no special status in those contexts. Only in geometry does 90 have a uniquely important connection to a right angle. Therefore those alternative choices fail the requirement of being a widely known, specific fact and not just an invented statement.

Common Pitfalls:
Students sometimes see that days also begins with D and quickly create 90 days in a random arrangement, ignoring whether anyone actually uses that phrase. This habit of forcing a match without thinking about common usage can lead to wrong answers in many number and letters puzzles. It is important to ask yourself whether the final expanded sentence is something you have seen in textbooks, news or common speech, not just something that could exist in theory.

Final Answer:
The initials puzzle 90 D in RA expands to the geometric fact 90 degrees in a right angle.