Is complete reduction of relief of a high land mass possible?
No, complete reduction of relief of a high land mass is not possible. Even after extensive erosion by rivers, glaciers, or wind, some resistant remnants always remain standing above the general surface level.
In fluvial erosion, the divides between drainage basins are lowered until they are almost completely flattened, leaving a lowland of faint relief with some low resistant remnants called monadnocks standing out here and there. This resulting landform is called a peneplain (an almost plain), indicating that it is never truly completely flat.
Similarly, in desert areas, through parallel retreat of slopes, mountains get reduced leaving an inselberg as a remnant of the mountain. The high relief is reduced to low featureless plains called pediplains, but complete elimination never occurs.
Explanation
The textbook context clearly explains that erosion processes (fluvial, glacial, and desert) reduce high land masses to low relief plains, but always leave behind resistant remnants. The key terms mentioned are: monadnocks (remnants in peneplains), inselbergs (remnants in pediplains), and the concept of 'almost plain' (peneplain) - all indicating that complete reduction is not achieved. The word 'almost' and the presence of remnants are crucial evidence that total flattening doesn't occur.