In the sentence "She must ______ a great time at her birthday party", which option correctly completes the idea that she is very probably enjoying herself right now?

Introduction / Context:
This question tests your understanding of modal verbs and aspect in English. The sentence She must ______ a great time at her birthday party suggests a strong guess about what is happening at the present moment. The correct answer must therefore combine the modal must with an appropriate verb form that expresses an ongoing action now.

Given Data / Assumptions:

• The sentence is: She must ______ a great time at her birthday party.
• The context suggests that the party is happening now and the speaker is inferring that she is enjoying herself.
• Options: be having, has, be had, is having.

Concept / Approach:
When we use must to express a logical conclusion about what is happening at this moment, we often use must be + present participle (verb + ing). This pattern indicates a strong assumption about a current ongoing action. So, She must be having a great time means the speaker is almost sure that she is right now enjoying the party. Forms like must has or must is having are grammatically incorrect, and be had is the wrong combination.

Step-by-Step Solution:
Step 1: Recognise that must here expresses strong probability, not obligation.Step 2: To indicate an action happening now, combine must with be + verb-ing: must be having.Step 3: Test option A: She must be having a great time at her birthday party. This is a natural and correct sentence.Step 4: Test has and is having; must has and must is having would both be ungrammatical combinations, since must must be followed by the base form of a verb (be, have, go, etc.).Step 5: Test be had; She must be had a great time is incorrect both in tense and structure. Therefore, be having is the only valid option.

Verification / Alternative check:
Compare structures with a similar meaning: She is probably having a great time at her birthday party. She must be having a great time at her birthday party. In both, the idea is an ongoing action combined with a degree of certainty. Replacing be having with has would make She must has a great time, which is ungrammatical, or She must have a great time, which describes a general habit, not a present ongoing event. The progressive form matches the immediate context of the party.

Why Other Options Are Wrong:
Has is the third person singular of have and cannot directly follow must; the correct base form is have or be. Be had is a passive like structure that does not fit here and would also require different context. Is having cannot follow must because must is already the modal; you cannot stack two auxiliaries in that way without using the base form after must.

Common Pitfalls:
Students sometimes forget that modals must be followed by the base form of the verb and try to use has or is after must. A helpful rule is: must + be + -ing for strong guesses about what is happening now, and must + have + past participle for strong guesses about what happened in the past. Remembering these patterns will help you avoid such errors in tense and aspect questions.

Final Answer:
The correct way to express that she is probably enjoying the party right now is be having, giving She must be having a great time at her birthday party, so option A is correct.