  Using 281/pb of data collected with the CLEO-c detector at the psi(3770)
resonance, we have studied Cabibbo-suppressed decays of D mesons to final
states with two kaons. We present results for the absolute branching fractions
of the modes D^0 --> K^+ K^-, D^0 --> K^0_S K^0_S, and D^+ --> K^+ K^0_S. We
measure B(D^0 --> K^+ K^-) = (4.08 +- 0.08 +- 0.09) x 10^-3, B(D^0 --> K^0_S
K^0_S) = (1.46 +- 0.32 +- 0.09) x 10^-4, and B(D^+ --> K^+ K^0_S) = (3.14 +-
0.09 +- 0.08) x 10^-3. We also determine the ratio B(D^0 --> K^+ K^-)/B(D^0 -->
pi^+ pi^-) = 2.89 +- 0.05 +- 0.06. For each measurement, the first uncertainty
is statistical and the second uncertainty is systematic.
