This question tests your understanding of modal verbs and passive voice in English grammar. Let's analyze the sentence structure and meaning:

The sentence is: "These flowers _______________ twice a week, but I always forget to water them."

Step 1: Understand the context
The speaker is talking about flowers that require regular watering (twice a week), but admits to forgetting this responsibility.

Step 2: Analyze the grammatical requirements
Since "flowers" cannot water themselves, we need the passive voice (be + past participle). The flowers receive the action of watering.

Step 3: Evaluate the modal verbs
We need a modal verb that expresses necessity or obligation. Let's examine each option:

• "Should be watered" - This expresses recommendation or mild obligation in passive voice. Correct structure: modal + be + past participle.
• "Ought to have been watered" - This refers to past obligation that wasn't fulfilled, but the context is about regular present watering.
• "Needn't have been watered" - This means watering was unnecessary, which contradicts the context that flowers need regular watering.
• "Ought to water" - This is active voice, suggesting the flowers should water something, which doesn't make logical sense.

Step 4: Match meaning and grammar
Only "should be watered" correctly expresses present obligation in passive voice while maintaining logical meaning: the flowers need regular watering, but the speaker forgets to do it.

Final Answer: Should be watered

Related Grammar Concepts

Modal Verbs of Obligation: Should, ought to, and must express different levels of obligation or recommendation.

Passive Voice Structure: When using modal verbs with passive voice, the structure is: modal verb + be + past participle (e.g., should be done, must be completed).

Key Formulas/Rules

Modal + Passive Formula: Modal Verb + be + Past Participle

Examples:
- It should be done by tomorrow.
- The report must be submitted today.
- The car ought to be repaired soon.