Carlos is studying the characteristics of a fundamental force. the force is sometimes repulsive and sometimes attractive. which force could carlos be studying? a. gravitational or strong nuclear b. electromagnetic or weak nuclear c. weak nuclear or strong nuclear d. electromagnetic or gravitational

The answer to this question would be: b.electromagnetic or weak nuclear

The electromagnetic force can be repulsive or attractive because it was influenced by the magnetic charge. The same charge (positive with positive) will be repulsive but different charge will be attractive(negative and negative). The gravitational force will always be attractive, not repulsive. 
Weak nuclear can do both like electromagnetic force.