How do scientists decide when to change or modify their understanding of a particular topic?

they carry out experiments to understand it

Explanation:

Scientists raise hypotheses which are tested by experiments made under controlled conditions in order to explain a particular topic. When a hypothesis is confirmed by the experimental data, the evidence obtained from this experiment provides the basis to increase the scientific knowledge about a particular issue. In consequence, experimentation can be considered as a critical step in the scientific method and research aims to advance knowledge of a particular phenomenon by confirming a hypothesis, which must be testable (i.e. verifiable as a result of further experimentations).

it should be d) transitional fossils.

they show the gradualism of evolution throughout a period of time