Which is base and not alkali?(a)- NaOH (b)- KOH (c)- Fe{{(OH)}_{3}} (d)- N{{H}_{4}}OH

The term "alkali" typically refers to substances that are soluble in water and produce hydroxide ions (OH-) when dissolved in water. Alkalis are also called bases. Bases that are soluble in water are referred to as alkali, and those that are not soluble in water are often simply called bases.

In the given options:

(a) NaOH (sodium hydroxide) is an alkali/base because it is soluble in water and produces hydroxide ions when dissolved in water.
(b) KOH (potassium hydroxide) is also an alkali/base for the same reasons as sodium hydroxide.
(c) Fe(OH)3 (iron(III) hydroxide) is not an alkali/base because it is not soluble in water, and it does not produce hydroxide ions when dissolved in water.
(d) NH4OH (ammonium hydroxide) is an alkali/base because it is soluble in water and produces hydroxide ions when dissolved in water.

So, the base that is not an alkali in the given options is (c) Fe(OH)3.