To determine which value represents the proportion of all professional astronomers that is employed by a government laboratory, we would typically rely on statistical data or research findings. However, without specific figures or context provided regarding the employment distribution of professional astronomers, I cannot definitively assign a proportion.
If you have data or references related to this, please share. Otherwise, the correct choice would depend on the most accurate statistics available on the employment of professional astronomers at government laboratories. If you need a general estimate, common knowledge or reports may suggest that about one-fourth is often a reasonable estimate.
If you require further assistance, feel free to clarify or specify!