I am getting a complex solution for a physical quantity, how can I plot it. Should I plot the real part or modulus?

solution is,

C1*sqrt(4*tau^2*M^2*lambda^2+Q^2)*LegendreP((1/2*I)*sqrt(7)-1/2, I*sqrt(7), (2*I)*lambda*M*tau/Q)+C2*sqrt(4*tau^2*M^2*lambda^2+Q^2)*LegendreQ((1/2*I)*sqrt(7)-1/2, I*sqrt(7), (2*I)*lambda*M*tau/Q)

where C1 and C2 are integration constants.