'Later than' means 'after the specified time'.
Option (b) is incorrect as 'most' is not written with the adjective in comparative degree ('later'). It is written with a positive degree adjective.
'Later then' does not form a meaningful phrase. So, option (c) is incorrect.
Option (d) is incorrect as it is the case of double comparative. Double comparatives should be avoided. 'More' should be removed as 'later' in itself is the adjective of comparative degree.
So, the correct sentence is: The movie was released later than expected.
Hence, option (a) is the answer.
Note: · The Positive Degree is used to denote the mere existence of some quality of what we speak about. It is used when no comparison is made.
· The Comparative degree denotes a higher degree of quality than the positive and is used when two things are compared.
· The Superlative degree indicates the highest degree of quality and is used when more than two things (or sets of things) are compared.