Given that rank Q is 4, so it will have 4 linearly independent columns as well as 4 linearly independent rows ($\because$ row rank of a matrix = column rank of a matrix)
So, A is correct and accordingly B is false.
C is false as order of $QQ^{T}$ will be 5 x 5 and given that rank of Q is 4 i.e. < 5. So the matrix is $QQ^{T}$ will be singular and hence not invertible.
Similarly D is false as order of $Q^{T}Q$ will be 6 x 6 and the matrix is $Q^{T}Q$ will be singular and hence not invertible.