Answer:
 is the expression with value of 15 when n = 7.
 is the expression with value of 15 when n = 7.
Step-by-step explanation:
To find the expression whose value is 15, substitute the value of n = 7 in each given expression. 
Expression 1:

Substituting the value of n, 

Simplifying, 

Since the value of the expression is 8 which is not equal 15. 
Hence expression  does not have value of 15 when n = 7.
 does not have value of 15 when n = 7.
Expression 2:

Substituting the value of n, 

Simplifying, 

Since the value of the expression is 16 which is not equal 15. 
Hence expression  does not have value of 15 when n = 7.
 does not have value of 15 when n = 7.
Expression 3:

Substituting the value of n, 

Simplifying, 

Since the value of the expression is 14 which is not equal 15. 
Hence expression  does not have value of 15 when n = 7.
 does not have value of 15 when n = 7.
Expression 4:

Substituting the value of n, 

Simplifying, 

Since the value of the expression is 15 which is equal 15. 
Hence expression  has the value of 15 when n = 7.
 has the value of 15 when n = 7.