Theyhave been fightingone another since last week. Here, in this sentence, the underlined word is a/an:

সঠিক উত্তর: Verb Phrase

Verb Phrase
- A verb phrase (VP, also called a "verb group") consists of a main verb and its auxiliary verbs (including modals). নিচের উদাহরণগুলো ভালো করে লক্ষ্য করুন: 
- We have been working since 9am.
- I will be going to France next week.
- It may have been being repaired.

A phrase is one or more words that form a meaningful grammatical unit within a clause. (Not a complete sentence though)

দুই বা ততোধিক শব্দ মিলিয়ে সেই শব্দগুচ্ছটি যখন একটি নির্দিষ্ট  Parts of Speech এর অর্থ প্রকাশ করে, তখন তাকে Phrase বলে। 

Types of Phrases: 
There are seven main types of phrases in English, as below:
- Noun phrases. Ex: as a good friend// The tiny mouse. 
- Verb phrases. Ex: drives carefully // was reading    
- Adjective phrases. Ex: very cold and dark// very tall 
- Adverb phrases. Ex: quite slowly// Only occasionally 
- Prepositional phrases. Ex: in first place// on the table. 
- Conjunctional phrase. Ex Rita as well as Mita has come. 
- Interjectional phrase. Ex: What a pity! you have failed. 

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