To determine which statement is true for 'Dobereiner's triads', we need to understand what Dobereiner's triads are. Johann Wolfgang Dobereiner was a German chemist who, in 1817, proposed that elements could be grouped into sets of three, which he referred to as triads. The key characteristics of these triads are:

1. Triads were grouped in such a way that the elements were ordered by increasing atomic masses.
2. The atomic mass of the middle element was approximately the average of the atomic masses of the other two elements in the triad.

Let's analyze the given statements:

• Statement I: "Triads were in order of atomic masses."
• Statement II: "Atomic mass of the middle element was roughly the average of atomic masses of other two elements."

Both statements hold true for Dobereiner's triads based on the historical context and definition:

• Verification of Statement I: The triads were deliberately arranged in order of increasing atomic masses, thereby confirming this statement.
• Verification of Statement II: The fundamental characteristic of Dobereiner's triads was that the atomic mass of the second element was roughly the arithmetic mean of the first and third elements in the triad, validating this statement.

Since both statements accurately describe Dobereiner's triads, the correct answer is Both I and II.