Monosyllabic Words: These are words consisting of one syllable.
When a particular word is pronounced, the word should like it consists of only one vowel or we should get a one vowel sound when a word is pronounced. These types of words are called syllables.
The word "weight" when pronounced gives us a sound of one vowel is present whereas the other options "command", "able", and "biscuit" is pronounced consisting of two vowel sounds, therefore "weight" is the correct answer.