Lack of money can cause worrying _______ within a family. Is the answer: a) straits, or b) strains
It is true that «dire straits» are worrying. But I do not think «worrying straits» is the answer here. The lack of money causes «strains». The «strains» are worrying.
In my opinion, being in dire straits is a cliche indicating severe trouble or problems - often of a monetary nature. Dire means is awful or appalling - straits a constriction - as in the Straits of Gibraltar.
I would not split the two words up if I were you - I think that strains is a much better fit in the context of the sentence.
Looking at this again 4 months later, I see that your argument is strong; still, I find strains in the plural quite odd, and will be loyal to my initial opinion: straits--
plural noun
a difficult and troubled situation, especially because of financial problems: So many companies are in such dire/difficult straits that their prices