Abstract nouns
refer to things that cannot be perceived through any of our senses
(touch, see, taste, smell, hear). They are emotions or states of being.
For example, 'happiness', intelligence', etc.
Concrete nouns are the opposite of abstract: they can be perceived through the senses.
Option C: 'Friendship' is a relationship between two people. It cannot be seen or touched.
'Blessing' is a state of being blessed or protected. It too cannot be perceived through the senses.
Hence both nouns are abstract and option C is correct.
Options A, B and D: 'Friendship' cannot be seen, touched, heard, smelled or tasted. It can be seen between two people but it does not have a physical form.
Similarly, 'blessing' can also not be experienced through these senses.
Hence these options are wrong.