There are many natural and polite ways to reply when someone says "thank you." The classic and most common response is "You're welcome." Other responses vary depending on the tone, formality, and the relationship with the person thanking you:

• Standard polite replies: "You're welcome," "My pleasure," "No problem," "Don't mention it."
• Casual/informal replies: "No worries," "No problem," "Sure," "Anytime."
• Expressing happiness to help: "Glad to help," "Happy to do this for you," "It was the least I could do."
• Showing mutual appreciation: "Thank you," "I appreciate you saying that," "That means a lot."
• Offering ongoing support: "I'm here for you anytime you need," "Anytime."

These responses can be chosen depending on the context, such as professional settings where "You're welcome" or "It was my pleasure" fit well, or among friends where "No worries" or "Sure thing" might be more natural. In summary, the simplest way is "You're welcome," but having a range of responses lets you express gratitude, friendliness, or formality as appropriate.