In a sample of 9.66 x 10(6) B(B) over bar pairs collected with the CLEO det
ector we make the first observation of B decays to an eta (c) and a kaon. W
e measure branching fractions B(B+ --> eta K-c(+)) = (0.69(-0.21)(+0.26) +/
- 0.08 +/- 0.20) x 10(-3) and B(B degrees --> eta K-c degrees) = (1.09(-0.4
2)(+0.55) +/- 0.12 +/- 0.31) x 10(-3), where the first error is statistical
, the second is systematic, and the third is from the eta (c) branching fra
ction uncertainty. From these we extract the eta (c) decay constant in the
factorization approximation, f(eta), = 335 +/- 75 MeV. We also search for B
decays to a chi (c0) and a kaon. No evidence for a signal is found and we
set 90% C.L. upper limits: B(B+ --> chi K-c0(+)) < 4.8 x 10(-4) and B(B<deg
rees> --> chi K-c0 degrees) < 5.0 x 10(-4).