In the MVT (Multiprogramming with a Variable number of Tasks) operating system, how many jobs can be run concurrently in main memory at a time?

Introduction / Context:
Historically, IBM mainframe operating systems introduced different multiprogramming schemes such as MFT (Multiprogramming with a Fixed number of Tasks) and MVT (Multiprogramming with a Variable number of Tasks). Exam questions often ask about the difference between these two schemes, especially about how many jobs they can keep in memory at once. This question focuses on MVT and the fact that it does not restrict the system to a fixed, hard coded number of jobs; instead, the number of simultaneously resident jobs depends on memory and partitioning.

Given Data / Assumptions:

• MVT stands for Multiprogramming with a Variable number of Tasks.

• It is a historical operating system concept for mainframes.

• The system supports multiprogramming by keeping more than one job in memory.

• Memory is divided into regions that can change size to fit different jobs.

Concept / Approach:
In MFT, memory is divided into a small number of fixed partitions, so only as many jobs as there are partitions can be resident at any given time. In contrast, MVT uses variable sized regions that can be created and adjusted to fit incoming jobs more flexibly. Therefore, the number of jobs that can be present in memory at once is not a simple constant but depends on job sizes and how memory is allocated. The correct option should emphasize that the number of concurrent jobs is variable and constrained by available memory rather than by a fixed count defined in advance.

Step-by-Step Solution:
Step 1: Recall the full form: MVT means Multiprogramming with a Variable number of Tasks. Step 2: Understand that tasks or jobs are placed into memory regions that can grow or shrink or be created and destroyed as needed. Step 3: Note that this prevents the system from being locked into a fixed number of partitions and allows more flexible use of memory. Step 4: Compare each option and remove any option that mentions a fixed number of jobs such as one, two or some unchangeable constant. Step 5: Choose the option that clearly states that the number of concurrent jobs is variable and is primarily limited by available main memory and current allocation.

Verification / Alternative check:
To verify, imagine a system with MVT where small jobs are loaded. More jobs can fit into memory at the same time compared to a case where very large jobs occupy most of the memory. This variation is the whole point of having a variable number of tasks. If the number were fixed, the system would behave more like MFT. Therefore, an answer that mentions variability and memory dependence aligns with the definition and purpose of MVT and is the correct one to choose.

Why Other Options Are Wrong:
Option B is wrong because running exactly one job at a time is a single task system, not multiprogramming at all. Option C is wrong because there is no architectural rule in MVT that restricts the system to exactly two jobs; that is an artificial limitation. Option D is wrong since MVT specifically aims to avoid a fixed number of tasks and instead allows the number of resident jobs to change when memory allocation changes.

Common Pitfalls:
Learners sometimes confuse MVT and MFT and assume both use fixed numbers of tasks. Remember that the key difference is right in the name: MVT emphasizes a variable number. Another pitfall is to think that there must be a specific numeric answer, such as four or five jobs. In reality, the correct answer is conceptual and focuses on the fact that the number of jobs is limited by memory and allocation policies, not by a small constant baked into the operating system. Keeping this conceptual focus in mind helps when facing similar theoretical operating system questions.

Final Answer:
Therefore, in MVT a variable number of jobs, limited mainly by available main memory and partitioning rather than a fixed constant, can be run concurrently in memory at a time.