Let \(a\), \(b\), \(c\), and \(d\) be real numbers. Given that \(ac=1\), \(\dfrac{b+c}{d}\) is undefined, and \(abc=d\), which of the following must be true?

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For a fraction to be undefined the denominator must be \(0\).

$$ \frac{b+c}{d} \text{ is undefined if } d=0 $$

If \(d=0\),

$$ abc = d $$ $$ abc=0 $$

We are told that \(ac=1\). Substituting:

$$ abc = 0 $$ $$ acb =0 $$ $$ (1)b=0 $$ $$ b= 0 $$