We know there are 26 alphabets in English. The 21st letter in the alphabet is 'U'.

We have 5 vowels and 21 consonants.

Vowels :: A, E, I, O, U.

U is not consonant and is a vowel.

Hence, the given statement that 21st letter in the alphabet i.e, U is consonant is a FALSE.

Here Pen is a tool which is used to write and poet uses it. Then, the needle is a tool which is used to stitch and it is used by Tailor. Except tailor, none of the other options is a person to use it.

Hence, Pen : Poet :: Needle : Tailor

The One-word substitute for 'a post without remuneration' is Honorary.

Cronyism is the practice of partiality in awarding jobs and other advantages to friends, family relatives or trusted colleagues, especially in politics and between politicians and supportive organizations.

Because the refractive index of glass is highest, light travels slowly in Glass. 

We know that the speed of light depends on the refraction index. The higher the index of refraction is, the slower the speed of light is.

R.I of air and vacuum = 0 (air is a teeny bit more but can be taken as 0)
R.I of water = 1.33
R.I of glass = 1.4.

Hence, Light travels slowest in Glass.

We fought against England in the Revolutionary War for Independence from the British rule.

Accoustics is a subject deals with the Study of sound

Aesthetics is a Philosophy of fine arts

The science or art of flight is called Aeronautics

The science of soil management is called Agronomy.