The supervisor is available to respond to any inquiries regarding the updated project guidelines.Here 'regarding' is a/an -

Regarding (preposition): 
English Meaning - In relation to; about. 
Bangla Meaning - সম্পর্কে; বিষয়ে।

• The supervisor is available to respond to any inquiries regarding the updated project guidelines.
- As a preposition, 'regarding' connects the noun 'inquiries' to the noun phrase 'the updated project guidelines', indicating the relationship between them. 
- এখানে 'regarding' হচ্ছে Participle preposition. 

Participle preposition: 
- কতগুলো Present participle এবং past participle- preposition এর মত ব্যবহৃত হয়ে থাকে, এদের Participle preposition বলে। 
- যেমন: Regarding this matter I know nothing. 

Source: A Passage to the English Language by S.M. Zakir Hussain.