The glass is full . . . . . . . . milk. A. of B. on C. by D. at

The correct answer is “of”.

The preposition “of” is used to indicate the contents of a container. In this case, the glass is full of milk.

The other prepositions do not make sense in this context.

• The preposition “on” is used to indicate location on a surface. For example, “The book is on the table.”
• The preposition “by” is used to indicate the means by which something is done. For example, “The house was built by the Smiths.”
• The preposition “at” is used to indicate a location or position. For example, “The meeting is at 2pm.”