First Principles Thinking: covariance formula
A is correct. Covariance between two asset returns equals the product of the correlation coefficient ($\rho_{12}$) and the standard deviations of the two assets ($\sigma_1$ and $\sigma_2$). This relationship, $Cov(R_1, R_2) = \rho_{12}\sigma_1\sigma_2$, is explicitly stated in the CFA Curriculum.
B is incorrect because the formula uses standard deviations ($\sigma_1, \sigma_2$), not variances ($\sigma_1^2, \sigma_2^2$). Using variances would overstate the covariance.
C is incorrect because covariance is a product relationship involving correlation and standard deviations, not a simple sum of standard deviations.