The correct option is C heartening
Options D and A: 'Heartening' means 'to make more cheerful or confident in an encouraging way'. The meaning is agreeable here: 'It is encouraging to know that the government has decided to revise the salaries of railway linesmen.
Hence option D is correct.
'Hearten' means 'to make more cheerful'. It means the same as the first word 'heartening', but their forms are different. The former is a present participle, which means that it functions as an adjective; it complements the subject 'it'.
On the other hand, 'hearten' refers to the action of making someone cheerful. Verbs cannot complement/modify nouns. Hence option A is incorrect.
Option B: 'Hearty' means 'wholesome and substantial'.
Option C: 'Heartfelt' means 'genuine' or 'sincere'.
These words are not very suitable for this sentence.
They form sentences that have an ambiguous meaning. Hence options B and C are incorrect.