In desktop PC hardware, which motherboard form factor typically uses a single 20-pin ATX main power connector from the power supply?

Introduction / Context:
Motherboards follow standard form factors that define their physical dimensions, mounting hole positions, and power connector types. When learning basic computer hardware, students are often asked to match well known form factors such as AT, Baby AT, and ATX with the style of power connectors they use. This question focuses on which form factor typically uses a single 20 pin ATX main power connector from the power supply unit, a very common arrangement in older and many classic desktop systems.

Given Data / Assumptions:
- The context is standard desktop PCs rather than servers or laptops.
- We are comparing AT, Baby AT, and ATX motherboard form factors.
- The connector mentioned is a single 20 pin ATX main power connector from the power supply unit.
- We assume common industry practice for classic designs, not unusual custom hardware.

Concept / Approach:
Early PC motherboards such as full size AT and Baby AT used a pair of power connectors known as P8 and P9. These older supplies were not called ATX supplies and did not use a single keyed plastic connector with 20 lines. In contrast, the ATX standard introduced a new main power connector that combined all required power lines into one rectangular 20 pin plug, later extended to 24 pins in newer revisions. Therefore, when the question refers to a single 20 pin connector for the main motherboard power, it is effectively pointing at the ATX standard, not at the older AT or Baby AT boards.

Step-by-Step Solution:
Step 1: Recall that original AT motherboards used two inline connectors from the power supply, often labelled P8 and P9, rather than one combined plug. Step 2: Remember that Baby AT is a smaller variation of the AT style, and it still typically uses the same pair of connectors, not a single 20 pin block. Step 3: Note that the ATX specification introduced a new style of power supply and connector, where the main power feeds are provided through one keyed 20 pin connector that locks into the ATX motherboard. Step 4: Compare this with the options. The only form factor traditionally associated with a single 20 pin ATX power plug is the ATX motherboard. Step 5: Conclude that ATX form factor motherboard is the correct answer.

Verification / Alternative check:
A simple way to verify this is to look at pictures or diagrams of classic AT and ATX boards. AT boards clearly show two separate connectors placed side by side, while ATX boards show a single long connector often near the right edge of the board. Technical references that describe the transition from AT to ATX mention the new 20 pin connector as a characteristic feature of ATX power supplies. This visual and documentary evidence confirms that the 20 pin connector belongs to ATX, not to AT or Baby AT.

Why Other Options Are Wrong:
Baby AT form factor motherboard: Uses the older dual connector arrangement compatible with AT power supplies, not a single 20 pin ATX plug.
Full size AT form factor motherboard: Also uses the P8 and P9 style connectors, so it does not match the 20 pin ATX connector description.
All of these motherboard types: This would be correct only if all form factors shared the same connector style, which is not the case here, so this option is incorrect.

Common Pitfalls:
Learners sometimes assume that any desktop motherboard uses an ATX style connector because modern systems commonly do, and they forget the older AT era. Others may pick the all of these option because they have seen adapters that allow various supplies to be mixed with different boards. The key is to focus on the standard and typical design for each form factor, not unusual conversions. Keeping a simple history in mind, where AT and Baby AT came first and ATX later introduced the 20 pin plug, helps avoid this confusion.

Final Answer:
The motherboard form factor that typically uses a single 20 pin ATX main power connector is the ATX form factor motherboard.