A two-stroke engine can be identified by the absence of valves. Let's understand why this is the correct answer in detail.

1. Introduction:
A two-stroke engine is a type of internal combustion engine that completes a power cycle with two strokes of the piston, compared to a four-stroke engine that requires four strokes. Two-stroke engines are commonly used in applications where a compact and lightweight design is required, such as motorcycles, chainsaws, and small boats.

2. Operating Principle:
The operating principle of a two-stroke engine involves the compression, ignition, and exhaust processes within two strokes of the piston. This is achieved by combining the intake and exhaust processes in a single stroke, eliminating the need for separate intake and exhaust valves.

3. Absence of Valves:
In a four-stroke engine, the intake and exhaust processes are controlled by valves, which open and close at specific times to allow the flow of air/fuel mixture into the cylinder and the expulsion of exhaust gases. However, in a two-stroke engine, these valves are not present.

4. Port Design:
Instead of valves, a two-stroke engine uses a unique port design to control the flow of air/fuel mixture and exhaust gases. The cylinder has ports located at specific positions, which are passages for the intake and exhaust gases. These ports are uncovered and covered by the movement of the piston.

5. Piston Movement:
As the piston moves up and down in the cylinder, it uncovers and covers the ports. During the downward stroke, the piston uncovers the intake port, allowing the air/fuel mixture to enter the cylinder. Simultaneously, the exhaust port is closed. During the upward stroke, the piston covers the intake port and uncovers the exhaust port, allowing the exhaust gases to be expelled.

6. Advantages of Absence of Valves:
The absence of valves in a two-stroke engine simplifies the design and reduces the overall weight and size of the engine. It also eliminates the need for a camshaft and associated components, further enhancing compactness and reducing cost.

7. Disadvantages of Absence of Valves:
However, the absence of valves in a two-stroke engine also leads to certain disadvantages. It results in less efficient scavenging of exhaust gases, which can reduce power output and increase emissions. Additionally, the absence of valves limits the ability to control the timing and duration of the intake and exhaust processes, which can affect engine performance and efficiency.

In conclusion, the absence of valves is the key characteristic that can be used to identify a two-stroke engine. Instead of valves, a two-stroke engine uses a unique port design and the movement of the piston to control the flow of air/fuel mixture and exhaust gases.