The correct answer is: B. L.M.T. + East longitude in time.
Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London. It is the time that is used as the basis for civil time in many countries around the world.
Local Mean Time (LMT) is the mean solar time at a particular location on Earth. It is calculated by averaging the time of noon at that location over a period of one year.
The difference between LMT and GMT is equal to the longitude of the location in time. For example, if a location is 15 degrees east of Greenwich, then its LMT is 1 hour ahead of GMT.
Therefore, the GMT corresponding to a given mean time is equal to the LMT plus the east longitude in time.
Option A is incorrect because it subtracts the east longitude in time. This would result in a time that is behind GMT.
Option C is incorrect because it subtracts the west longitude in time. This would result in a time that is ahead of GMT.
Option D is incorrect because it is not one of the possible answers.