Fill in the blank with a suitable modal verb:
Congratulations on passing your exam. You ______ be very pleased.
(a)will
(b)can
(c)must
(d)could

Hint: Look for pointers suggesting which modal auxiliary to use. The action has occurred in the given sentence, i.e. it occurred in the past. These pointers are indicators of certain situations.

Complete answer: A modal verb is a type of verb that is used to indicate
modality – that is: likelihood, ability, permission, request, capacity, suggestions, order,
obligation, or advice. We need to find the verb from the options that indicates possibility/likelihood.

We can discern whether the actions are happening right now by certain pointers that are
given in the sentence in the question. So, let us deconstruct the given sentence.
In the first part, the sentence is telling us that the person is congratulating someone who has just passed their exam. The second part of the sentence suggests that the person is telling them that they are pleased. This means that we need a modal verb that indicates possibility/likelihood.

Now, we know exactly what we are looking for. Let’s look at the given options:
(a)will - This modal auxiliary is most often used to express a strong desire. This does not fit into the context of the given sentence as it does not indicate possibility. This is not the required answer. So, this is an incorrect option.
(b)can - This modal auxiliary is most often used to express a person or thing’s ability to do something. This does not fit into the context of the given sentence as it does not indicate possibility. This is not the required answer. So, this is an incorrect option.
(c)must - This modal auxiliary is most often used to express a possibility. This fits into the context of the given sentence as it does indicate possibility. This is the required answer. So, this is the correct option.
(d)could - This modal auxiliary is most often used to express past ability. This does not fit into the context of the given sentence as it does not indicate possibility. This is not the required answer. So, this is an incorrect option.

Note: Choose the most appropriate option given the context of the sentence. Make sure all the forms of verbs are aligning with each other and work with each other perfectly.