Which of the following is primarily focused on the protection of children's rights?

The focus of the correct choice lies in its dedicated mission to uphold and protect the rights of children globally. This organization operates under the principles set forth in the United Nations Convention on the Rights of the Child, advocating for children's access to essential services such as education, healthcare, and protection from violence and exploitation.

UNICEF works tirelessly to ensure that every child has the opportunity to thrive and fulfill their potential, emphasizing not only survival but also development and participation in decisions that affect their lives. This includes addressing issues such as child labor, education inequality, and malnutrition, which are crucial for promoting the overall welfare of children.

In contrast, other organizations mentioned, while they may serve important global health or developmental roles, do not specifically prioritize children’s rights in the same primary manner. For instance, SPANA focuses on the welfare of animals, Heifer International is dedicated to alleviating poverty through sustainable agricultural programs, and WHO's broad mandate is aimed at public health but does not specifically center on children’s rights.

Thus, the distinctive mission of UNICEF sets it apart as the foremost entity devoted entirely to advocating for and safeguarding the rights and well-being of children globally.