Determine the [oh] concentration in a 0.169 m ca(oh)2 solution. 0.338 m 0.169 m 5.92 x 10-14 m 2.96 x 10-14 m 0.298 m

0.338
Explanation: 
Ca(OH)2 , also known as calcium hydroxide, is a strong base although it is not very soluble in water.
Therefore the concentration of 1 OH is equal to 0.169 m,
Since Ca(OH)2 contains two OH, therefore, concentration of O[OH] is given as:
[OH] = 2 x 0.169 = 0.338 m 

The answer is a) carbon-12

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.