Answer: a. ineffective branding.
Step-by-step explanation:
People were already used to seeing Bic as a pen brand to the extent that in some countries Bic is a generic name for all pens . When they introduced the underwear, people could probably see no need to buying an underwear with the Bic brand because they knew it as a pen brand. They therefore could not generate a strong brand association with the new product.
It is the same logic as when an actor has been on a Sitcom for so long that when they take on other roles, the public judges them a bit too harshly for it because they are not used to them in other roles.