This question tests your understanding of verb tenses and passive voice in English grammar. The correct choice depends on the context of the sentence, which is implied by the options.

The blank (64) requires a verb form. Let's analyze the options:

Step 1: Identify the Voice
First, determine if the sentence requires an active or passive voice. The subject of the sentence (which is missing but implied) is either doing the action (active) or receiving the action (passive).

• "is checking" and "check" are in the active voice.
• "checked" and "is checked" are in the passive voice.

Step 2: Identify the Tense
Next, look at the verb forms to understand the time reference.

• "check" is the base form, used for the simple present tense (active).
• "is checking" is the present continuous tense (active).
• "checked" is the simple past tense. It can be active ("I checked") or the past participle used for passive voice ("it was checked").
• "is checked" is the simple present tense in the passive voice.

Step 3: Apply Context
Without the full sentence, we must rely on the most common and logical usage. The phrase "is checked" is a standard construction for describing a routine procedure or a general rule in the present tense. For example: "The system is checked every morning for errors." This implies a regular, passive action.

Final Answer: The most appropriate option is is checked. It correctly uses the simple present passive voice, which is commonly used to describe standard procedures or states of being.

Related Topics & Formulae

Passive Voice Structure:
The general formula for the passive voice is:
$\text{Form of 'to be'}+\text{Past Participle}$

Examples:

• Present Simple: is/are + past participle (The report is written).
• Past Simple: was/were + past participle (The window was broken).
• Present Continuous: is/are being + past participle (The car is being repaired).

Key Theory: Use the passive voice when the action is more important than who performed it, or when the doer is unknown or obvious.