The Canadian Red Ensign served as the Canadian flag for about:

Discover Canada records this in one direct sentence about the Canadian Red Ensign. The guide writes: The Canadian Red Ensign served as the Canadian flag for about 100 years. The duration the test wants is therefore about 100 years.

Two precise commitments. Discover Canada commits the Canadian Red Ensign to TWO specific facts: (1) it served as the Canadian flag — the country's national flag for that period; (2) the duration was about 100 years. So the Red Ensign covered roughly the entire first century of Canadian Confederation, from 1867 forward.

The current flag replaced the Red Ensign in 1965. Discover Canada commits the modern flag's first raising to a specific year: "A new Canadian flag was raised for the first time in 1965. The red-white-red pattern comes from the flag of the Royal Military College, Kingston, founded in 1876." So the current red-and-white maple-leaf flag took over from the Red Ensign in 1965 — meaning the Red Ensign was Canada's national flag for roughly the period from Confederation in 1867 (about 100 years earlier) until the maple-leaf flag's first raising in 1965.

The colour heritage predates both flags. Discover Canada commits Canada's national colours to a much older tradition: "Red and white had been colours of France and England since the Middle Ages and the national colours of Canada since 1921." So while the named flags changed in 1965, Canada's red-and-white identity continued. The Discover Canada guide also notes that "the Union Jack is our official Royal Flag" — meaning Canada has BOTH a national flag (today's maple-leaf flag) AND a Royal Flag (the Union Jack), each with a distinct role. The Red Ensign featured the Union Jack in the upper-left and the Canadian arms on the right — a design that connected Canada's British heritage with its emerging national identity. Each provincial and territorial government also has its own flag "that embody their distinct traditions." So when the test asks how long the Canadian Red Ensign served as the Canadian flag, the source-precise answer is about 100 years — roughly the country's first full century from Confederation to the 1965 first raising of today's maple-leaf flag.

The question is testing whether new citizens know how long the Canadian Red Ensign served as the national flag. Discover Canada commits to one figure: about 100 years. The right test answer matches that.

The wrong answer choices each substitute a different period. The first choice — 50 years — is half the source's named figure. The second choice — 75 years — is also below the source's named figure. The fourth choice — 150 years — exceeds the source's named figure. Only about 100 years — the source's exact named duration — matches.