Identify the matrix that can be a state transition matrix.

A. \[\phi = \left[ {\begin{array}{*{20}{c}} {{e^{ - t}}}&0\\ 1&{{e^{ - 2t}}} \end{array}} \right]\]

B. \[\phi = \left[ {\begin{array}{*{20}{c}} {{e^t}}&t\\ 1&{2{e^{ - t}}} \end{array}} \right]\]

C. \[\phi = \left[ {\begin{array}{*{20}{c}} {{e^t} + {e^{ - t}}}&0\\ 0&{2{e^{ - t}}} \end{array}} \right]\]

D. \[\phi = \left[ {\begin{array}{*{20}{c}} {{e^{ - t}}}&0\\ 0&{{e^{ - 2t}}} \end{array}} \right]\]

Answer: Option D