Question

in order to observe your crystals as they grow, you...
use your estimation skills to make general observations.
use your intuition.
use your five senses.
use your four senses.

Explanation:

To observe crystals growing, one uses the five senses (sight to see their shape and color, touch to feel texture if possible etc.). Estimation skills are more for numerical - related approximations, intuition is more of a gut - feeling and not a reliable scientific observation method, and we have five senses for general observation in science, not four.

Answer:

use your five senses.