Step 1: Define cohesiveness.
Cohesiveness is the degree of attraction and commitment members feel toward the group.
Step 2: Factors.
Attractiveness of group/activities, investment of effort (effort justification), and external threat/danger (which fosters unity against an out-group) all increase cohesion.
Step 3: Conclude.
Each option is a recognized antecedent; thus “All of these.”
\[
\boxed{\text{Option D}}
\]