Questions

Envelope is a letter holder for one letter and in the question it mean that it holds one letter. Eye is also a word which starts and end with E but has one letter in the middle.

Sum of numbers in the outer circle in before Arc should come in inner circle patterns in the clock wise direction next Arc.

i.e, 9+6 = 15 comes in next arc of inner circle

7+3 = 10 comes in next arc of inner circle

In the same way, 4+2 = 6 comes in place of '?'.

Facilitate means assist someone in their progress or work. Here Help is the only word which is similar in meaning with facilitate. Halt and check are antonyms i.e opposite in meaning of facilitate. 

Hence, facilitate is similar in meaning to help.

In 1814, George Stephenson developed steam engine to haul coal from mines to ports by railways. In 1830, the first railway train began to carry passengers and freight from Liverpool to Manchester.

In Indian History, Dandi March or Salt Sathyagraha or The Salt March, also known as the Dandi Satyagraha, was an act of nonviolent civil disobedience in colonial India led by Mohandas Karamchand Gandhi to produce salt from the seawater. 

It is also called as 'White Flowing River'.