Solution
= Solution
{c}
The <countable chain condition for forcing> says that every <antichain in a forcing order> is a <countable set>. Equivalently,
$$
\boxed{\text{There is no uncountable set of pairwise incompatible conditions.}}
$$
In this question the assertion is evaluated inside $M$. Although every subset of the countable set $M$ is externally countable, that observation does not establish the internal chain condition: $M$ need not contain an enumeration of an antichain.