Which one of the following designations does not refer to a modern battle tank used by the armed forces of any country?

Introduction / Context:
Battle tanks are heavy armoured fighting vehicles that form the backbone of many modern armies. They are usually identified by a combination of letters and numbers, such as T-55, T-72 or T-90. Defence related examinations often test whether a candidate can distinguish between genuine tank designations and other military equipment that may sound similar. This question asks which designation does not signify a modern battle tank, so you must be able to recall and match specific model numbers with their correct platform.

Given Data / Assumptions:

• The options given are T-55, T-155, T-72 and T-90.
• T-55, T-72 and T-90 are well known Soviet or Russian origin main battle tanks used by several countries.
• One option is not a battle tank but something else, even though it looks similar in format.
• No extra numerical or technical data is provided, so the question is based on factual recognition.

Concept / Approach:
To solve this question, you should use direct factual knowledge of prominent tank models. Many of them, especially those from the Soviet Union and Russia, use the letter T followed by a number. However, other military platforms such as artillery guns or vehicles may also use similar looking codes. The correct approach is to identify which of the given codes is not associated with a main battle tank in standard defence literature. Once that is done, the remaining codes can be confirmed as genuine battle tanks, which helps to eliminate them as incorrect choices.

Step-by-Step Solution:
Step 1: Recall that T-55 is a widely known Soviet era main battle tank, introduced in the mid twentieth century and used by many countries. Step 2: Recall that T-72 is another famous main battle tank that succeeded the earlier T series tanks and has been exported to many nations. Step 3: Recall that T-90 is a modern Russian main battle tank derived from earlier T series designs and is also used by the Indian Army. Step 4: Notice that T-155 is not recognised as a main battle tank. Instead, the designation T-155 is associated with a self propelled artillery gun system developed by another country, which is a different category of weapon system. Step 5: Since only one option does not signify a battle tank, T-155 is the correct answer.

Verification / Alternative check:
An alternative way to verify your answer is to remember that when defence exams refer to Russian or Soviet tanks, they almost always mention T-54 or T-55, T-62, T-72, T-80 and T-90 as the major series. None of the standard tank lists include a T-155 battle tank. You may also recall that some artillery systems carry designations like T-155 or similar, but they are clearly described as howitzers or gun systems, not as main battle tanks. Once you remember this pattern, it confirms that T-55, T-72 and T-90 are all tanks while T-155 is not.

Why Other Options Are Wrong:

• T-55: This is a genuine Soviet era main battle tank and therefore signifies a battle tank.
• T-72: This is a widely used main battle tank that has served in many armies around the world, so it is not the odd option.
• T-90: This is a modern Russian main battle tank, also in service with India, and clearly signifies a tank.

Common Pitfalls:
A frequent mistake is to assume that any code beginning with the letter T and a number represents a tank. In reality, different countries use different coding schemes, and sometimes artillery or other weapons can have similar looking names. Another pitfall is to overthink the question and doubt well known tank names like T-72 or T-90 instead of focusing on the unfamiliar code. Candidates may also confuse artillery systems with tanks just because both belong to the armoured branch of the army, so it is important to recall their correct classification.

Final Answer:
The designation that does not signify a modern battle tank is T-155.