Four sentences are given below as A, B, C and D. Choose the sentence that is grammatically incorrect in describing how many Japanese cities had tram systems until the 1960s when increased motorization made lines disappear.

Introduction / Context:
This problem focuses on expressing time relationships clearly in complex sentences. The topic is tram systems in many Japanese cities and how increased motorization during the 1960s caused some lines to disappear. You are asked to identify the sentence that fails to express this timeline in acceptable English.

Given Data / Assumptions:
The time period is around the 1960s.
Many Japanese cities had extensive tram systems before or until increased motorization.
Increased motorization caused some lines to disappear.
Three options are grammatically acceptable, one is not.
You must select the grammatically incorrect sentence about this historical change.

Concept / Approach:
The main concepts are the correct use of until, when, and after to show the relationship between past conditions and changes. Phrases like until the 1960s, when and in the 1960s are natural ways to refer to the time of transition. However, combinations like after the 1960s until when are ungrammatical and confusing. The approach is to check whether the temporal markers are used in a clear and idiomatic way.

Step-by-Step Solution:
Step 1: Option A says Many Japanese cities had extensive tram systems until the 1960s, when increased motorization started to make some lines disappear. This clearly shows that the trams existed up to the 1960s and then motorization began to remove lines. It is correct. Step 2: Option B says Until the 1960s, when increased motorization started to make some lines disappear, many Japanese cities had extensive tram systems. This simply reverses the order of clauses but remains correct. Step 3: Option C says Many Japanese cities had extensive tram systems when increased motorization started to make some lines disappear in the 1960s. This indicates that both facts overlapped in time during the 1960s and is grammatically acceptable. Step 4: Option D says Increased motorization started to make some lines disappear after the 1960s until when many Japanese cities had extensive tram systems. Step 5: The phrase after the 1960s until when is not idiomatic English. After and until are being combined in a way that does not make sense, and until when is not a standard connector here. Step 6: Therefore option D is the grammatically incorrect sentence that should be selected.

Verification / Alternative check:
Try to repair option D. A correct version would be Until the 1960s, many Japanese cities had extensive tram systems, but increased motorization then started to make some lines disappear. The fact that you must remove after and until when to make it work confirms that the original phrasing in option D is wrong.

Why Other Options Are Wrong:
Option A is acceptable because until the 1960s, when is a standard way of connecting a time period with the event that follows it.
Option B is acceptable because it uses the same phrase until the 1960s, when in a different clause order but with correct grammar.
Option C is acceptable because it uses when and in the 1960s to indicate timing and cause without creating a contradiction.
Option D is incorrect because after the 1960s until when is an ill formed sequence of prepositions and conjunctions.

Common Pitfalls:
Many learners fail to notice that a sentence can be incorrect not because of agreement errors, but because of awkward or impossible combinations of time markers. Examiners often test the proper use of until, since, when, and after, especially in historical or process descriptions, so it is important to check how these words work together.

Final Answer:
The grammatically incorrect sentence is Option D. Increased motorization started to make some lines disappear after the 1960s until when many Japanese cities had extensive tram systems.