Question

Fill in the blank with a suitable preposition.
He had a photo of his family _____ the desk.
a) on
b) upon
c) of
d) for

Answer 1

Hint: Preposition talks about the position of the subject and tells us the relation of the subject with respect to the object. For example, The chocolates were in the big box. Here the subject is chocolates and the object is a box. The preposition “in” tells us the relation between the chocolates and the box: that the chocolates were in the box, not outside it, on it, or beside it.

Complete answer:
The correct answer is option (a). The subject is a photo and the object is a desk. A photo is placed on a desk. Therefore, the photo of his family was on his desk.
Option (b) is an incorrect answer because upon is a more formal version of “on”. Upon is used when describing intangible things such as ideas, principles, etc. He chanced upon an idea for his project work. An idea is an intangible thing.
Option (c) is incorrect because “of” is used when we usually want to say what a thing consists of. The dress is made of cotton. The phrase “photo of his family of the desk” makes no sense.
Option (d) is wrong because having a photo of his family for the desk means that the family photo was for the benefit of the desk which is illogical. Some examples of sentences where we use the preposition “for” are: He works hard for his family. She went out of his way for him. I would have been dead but for my friend who arrived in time to take me to the hospital.
Hence option (a) is correct. 

Note:
There is a chance to be confused between “on” and “upon”. As already explained, “upon” is used for more formal and abstract/intangible things such as feelings, ideas, principles, etc. It is not used for tangible things such as a desk, chair, table, shelf, etc.