Sentence:  This means that such factors as the health of the parents , particularly the mother , their <WSD>ability</WSD> to provide their children with the necessities of life , the degree of population density of a country and the shortage of housing facilities may legitimately be taken into consideration in determining the number of offspring . 
Meaning of the word marked with <WSD> tags:  the quality of being able to perform; a quality that permits or facilitates achievement or accomplishment