$A$ standing wave pattern of amplitude $A$ in a string of length $L$ shows $2$ nodes (plus those at two ends). If one end of the string corresponds to the origin and $v$ is the speed of the progressive wave,the disturbance in the string could be represented (with appropriate phase) as:

• A
  $y(x,t) = A\sin \left( {\frac{{2\pi x}}{L}} \right)\cos \left( {\frac{{2\pi vt}}{L}} \right)$
• B
  $y(x,t) = A\cos \left( {\frac{{3\pi x}}{L}} \right)\sin \left( {\frac{{3\pi vt}}{L}} \right)$
• C
  $y(x,t) = A\cos \left( {\frac{{4\pi x}}{L}} \right)\cos \left( {\frac{{4\pi vt}}{L}} \right)$
• D
  $y(x,t) = A\sin \left( {\frac{{3\pi x}}{L}} \right)\cos \left( {\frac{{3\pi vt}}{L}} \right)$