What is the ruling regarding a woman who vowed to fast for three months but then became unable to do so? Does expiation suffice, or is she obligated to fast intermittently? And is it permissible to fast the months separately?

It is disliked to make a vow, but whoever vows obedience to Allah must fulfill it. The woman must fulfill her vow by fasting the three months, even if they are separate, provided she did not intend consecutive fasting. It is not permissible for her to switch to expiation as long as she is able to fast. If she becomes unable to fast with no hope of recovery, an expiation for breaking an oath (kaffarat yameen) suffices. Some scholars hold the opinion that she must also feed the poor for each day of the three months in addition to the kaffarat yameen.