Q.If humans were to settle on other planets which of the fundamental quantities will be in trouble? Why?

Step 1. Among the seven fundamental quantities, length, mass, electric current, temperature, amount of substance and luminous intensity are all defined via UNIVERSAL physical standards (the speed of light, an atomic transition frequency, a fixed number of entities, etc.) that hold identically anywhere in the universe.

Step 2. Time, however, has everyday PRACTICAL units -- the 'day' (Earth's rotation period) and the 'year' (Earth's revolution period around the Sun) -- that are defined entirely by Earth's own motion.

Step 3. On another planet, both the rotation period and the orbital period are completely different from Earth's (e.g. a Martian day is about 24 hours 37 minutes, and a Martian year is about 687 Earth days) -- so 'a day' and 'a year' as practical measures of time would not carry over meaningfully. …

Identify which fundamental quantity's PRACTICAL unit is tie …

• Assuming mass or length would be the problem, when their SI definitions are already universal (light speed, atomic transitions), not Earth-specific. …