The error is in first part. ‘Tell’ should be replaced with ‘Ask’. Tell is a command whereas Ask is a request and hence more courteous in this case. As this is a request the sentence should start with ‘Ask’ rather than ‘tell’.

The given sentence is grammatically correct and there is no error.

‘prefer’ is a verb that is always followed by the preposition ‘to’ expressing choice or preference over something else. In this case choice of ‘this book’ over the ‘other’ one. ‘Than’ is used for comparing to options, ‘prefer to’ expresses choice. Correct expression will be ‘to that one’.

The given sentence is grammatically correct and there is no error.

The error is in first part. ‘One of the’ should always be followed by a plural noun, expressing that the speaker chooses one from many. Hence ‘features of travel’ is the correct expression, indicating that there are many interesting features about Himachal Pradesh but the most interesting one among them is the provision of large number of travelers’ lodges.

The error is in third part. By using ‘not’ after ‘unless’, both the negatives cancel each other. So, the correct usage will be ‘unless he reports for duty immediately’. Since “unless” is negative it should be
followed by affirmation, like “unless he reports for duty”. Incase “unless” was absent, we could have written “if he does not report” and expressed the same meaning.

The error is in first part. ‘Considering’ is a preposition that relates the facts to the principal and hence does not need to be followed by another preposition ‘about’. A preposition like ‘considering’ should be followed by a pronoun like ‘these’ in this case. The use of about is unnecessary and shall be omitted. The sentence indicates that the principal offered him a seat only after considering certain facts.

Error is in third part. Here, “there” is an adverb that qualifies the verb ‘is’. The position of ‘there’ in the sentence is incorrect. The correct usage should be ‘there is no trouble’. It indicates the absence of any trouble in a country.