Final answer:
The word 'monstrous' could be used to replace 'colossal gigantic' in the given excerpt as it conveys a similar sense of immense size.
Step-by-step explanation:
The word 'monstrous' could replace the phrase 'colossal gigantic' in the excerpt to reflect its meaning. Both 'colossal' and 'gigantic' suggest something of an immense size, which is similarly conveyed by the word 'monstrous.' While 'monstrous' often has a connotation of being large and frightening, in this context, it aptly communicates the overwhelming scale of the machinery. Options 'incomprehensible,' 'magnificent,' and 'extensive' do not align as closely in meaning; 'incomprehensible' implies something that cannot be understood, 'magnificent' leans towards splendor and exceptionality, and 'extensive' suggests a large area, scope, or duration but not necessarily the physical size or imposing nature of 'colossal gigantic.'