The longest DNA molecule is: ((MCAER 2014))

✅Explanation: A null matrix, also known as a zero matrix, is a matrix in which all elements are zero. It is often denoted by the symbol O.

📌Other Options Explanations:
-B. An identity matrix is a square matrix with 1s on the diagonal and 0s elsewhere.
-C. A diagonal matrix is a square matrix where all non-diagonal elements are zero.
-D. A scalar matrix is a diagonal matrix where all diagonal elements are equal.

🔑Key Points:
Zero Matrix: 
-A zero matrix that has all its elements equal to zero.
-Since a zero matrix contains only zeros as its elements, therefore, it is also called a null matrix.
-The m × n matrix whose elements are all zeroes is called the Null matrix.
-A zero matrix is denoted by 'O'. It is an additive identity matrix that results in the same matrix when added to a matrix of order m × n.
∴ The m x n matrix whose elements are all zeroes is called the Null matrix.