The differential equation of the family of curves y = p cos (ax) + q sin (ax), where p, q are arbitrary constants, is

• A. \(\frac{{{d^2}y}}{{d{x^2}}} - {a^2}y = 0\)
• B. \(\frac{{{d^2}y}}{{d{x^2}}} - ay = 0\)
• C. \(\frac{{{d^2}y}}{{d{x^2}}} + ay = 0\)
• D. \(\frac{{{d^2}y}}{{d{x^2}}} + {a^2}y = 0\)