The correct option is
B as tall as
The given sentence indicates that the subject's height is equal to his father. The answer must thus, be an adjective of the positive degree and must indicate the above meaning.
Option A is incorrect. Here the phrase is in positive degree but, 'than' can only used with the comparative degree. Thus, the phrase itself is grammatically incorrect.
Option B is correct. 'as tall as' indicates that the subject's height is the same as his father. Also, it is a correct positive degree phrase. Thus, it is the correct answer.
Option C is incorrect. Here the phrase is in the positive degree but, 'than' can only used with comparative degree. Thus, the phrase itself is grammatically incorrect.
Option D is incorrect. In 'so tall as' the conjunction 'so...as' is used only to indicate reason. Since the sentence does not indicate reason, option D does not convey the correct meaning and is incorrect.